Leave out of the build context what the app's .dockerignore names (closes #274)

Docker does not apply .dockerignore to a build context sent as a tar,
so upaas sent every file in the clone. The build now reads the ignore
file as docker build does, <Dockerfile>.dockerignore next to the
Dockerfile if there is one, otherwise .dockerignore at the root, with
the ignorefile reader of moby/patternmatcher, and passes its patterns
as exclude patterns. As the docker command line does, the Dockerfile
and .dockerignore are never left out. The ignore file is read through
os.Root, so it cannot be read from outside the clone. An ignore file
that cannot be read, or a malformed pattern, fails the build.

Model: opus-5-5

README.md

@@ -268,6 +268,12 @@ upaas fails the deploy instead of building. A Dockerfile that uses
 `RUN --network` needs Docker Engine 23.0 or later unless its `# syntax=` line
 names Dockerfile frontend 1.3 or later, such as `docker/dockerfile:1`.
 
+The build context leaves out the files the app's ignore file names, as
+`docker build` does: `<Dockerfile>.dockerignore` next to the app's Dockerfile if
+there is one, otherwise `.dockerignore` at the root of the repository. The
+Dockerfile and `.dockerignore` are always sent, even when the ignore file names
+them.
+
 Session secrets are automatically generated on first startup and persisted to
 `$UPAAS_DATA_DIR/session.key`.
 

TODO.md

@@ -20,6 +20,13 @@ regress.
 
 # Completed Steps
 
+- 2026-10-03: An app's build context leaves out the files its `.dockerignore`
+  names, such as `.git/config`, as `docker build` does; before, every file in
+  the clone was sent. An ignore file next to the Dockerfile,
+  `<Dockerfile>.dockerignore`, is read instead when there is one. The Dockerfile
+  and `.dockerignore` are always sent. An ignore file that cannot be read, or
+  holds a pattern Docker rejects, fails the build (#274).
+
 - 2026-10-02: In a window too narrow for the top bar, such as 390 px, the New
   App and Logout buttons move to a second row instead of running into "by
   @sneak"; the bar keeps a gap between its two sides at every width (#272).

go.mod

@@ -15,6 +15,7 @@ require (
 	github.com/joho/godotenv v1.5.1
 	github.com/mattn/go-sqlite3 v1.14.32
 	github.com/moby/buildkit v0.16.0
+	github.com/moby/patternmatcher v0.6.0
 	github.com/oklog/ulid/v2 v2.1.1
 	github.com/prometheus/client_golang v1.23.2
 	github.com/spf13/viper v1.21.0
@@ -60,7 +61,6 @@ require (
 	github.com/klauspost/compress v1.18.2 // indirect
 	github.com/moby/docker-image-spec v1.3.1 // indirect
 	github.com/moby/locker v1.0.1 // indirect
-	github.com/moby/patternmatcher v0.6.0 // indirect
 	github.com/moby/sys/sequential v0.6.0 // indirect
 	github.com/moby/sys/signal v0.7.1 // indirect
 	github.com/moby/sys/user v0.4.0 // indirect

internal/docker/buildcontext.go

@@ -0,0 +1,94 @@
+package docker
+
+import (
+	"errors"
+	"fmt"
+	"io"
+	"io/fs"
+	"os"
+	"path/filepath"
+
+	"github.com/docker/docker/pkg/archive"
+	"github.com/moby/patternmatcher"
+	"github.com/moby/patternmatcher/ignorefile"
+)
+
+// defaultDockerfileName is the Dockerfile Docker builds when none is named.
+const defaultDockerfileName = "Dockerfile"
+
+// defaultDockerignoreName is the ignore file at the root of a build context,
+// read when the Dockerfile has no ignore file of its own.
+const defaultDockerignoreName = ".dockerignore"
+
+// tarBuildContext returns a tar of the build context in contextDir that leaves
+// out the files the app's ignore file names, as docker build does; Docker does
+// not apply the ignore file to a build context sent as a tar. dockerfile is
+// the path of the Dockerfile inside contextDir.
+func tarBuildContext(contextDir, dockerfile string) (io.ReadCloser, error) {
+	excludes, err := readDockerignore(contextDir, dockerfile)
+	if err != nil {
+		return nil, err
+	}
+
+	return archive.TarWithOptions(contextDir, &archive.TarOptions{
+		ExcludePatterns: excludes,
+	})
+}
+
+// readDockerignore returns the patterns of the files to leave out of the
+// build context in contextDir, read as docker build reads them for the
+// Dockerfile at dockerfile: from <dockerfile>.dockerignore next to the
+// Dockerfile if there is one, otherwise from .dockerignore at the root of the
+// context. Without either file there are no patterns.
+func readDockerignore(contextDir, dockerfile string) ([]string, error) {
+	if dockerfile == "" {
+		dockerfile = defaultDockerfileName
+	}
+
+	// Reading through os.Root keeps the read inside the build context, even
+	// when the ignore file is a symlink.
+	root, err := os.OpenRoot(contextDir)
+	if err != nil {
+		return nil, err
+	}
+
+	defer func() { _ = root.Close() }()
+
+	name := dockerfile + defaultDockerignoreName
+
+	file, err := root.Open(name)
+	if errors.Is(err, fs.ErrNotExist) {
+		name = defaultDockerignoreName
+		file, err = root.Open(name)
+	}
+
+	if errors.Is(err, fs.ErrNotExist) {
+		return nil, nil
+	}
+
+	if err != nil {
+		return nil, fmt.Errorf("failed to read %s: %w", name, err)
+	}
+
+	defer func() { _ = file.Close() }()
+
+	excludes, err := ignorefile.ReadAll(file)
+	if err != nil {
+		return nil, fmt.Errorf("failed to read %s: %w", name, err)
+	}
+
+	// Like the docker command line, never leave out .dockerignore or the
+	// Dockerfile: Docker reads the Dockerfile from the context.
+	for _, keep := range []string{defaultDockerignoreName, filepath.ToSlash(dockerfile)} {
+		excluded, err := patternmatcher.MatchesOrParentMatches(keep, excludes)
+		if err != nil {
+			return nil, fmt.Errorf("invalid pattern in %s: %w", name, err)
+		}
+
+		if excluded {
+			excludes = append(excludes, "!"+keep)
+		}
+	}
+
+	return excludes, nil
+}

internal/docker/buildcontext_test.go

@@ -0,0 +1,237 @@
+package docker //nolint:testpackage // tests the unexported performBuild
+
+import (
+	"archive/tar"
+	"errors"
+	"io"
+	"log/slog"
+	"net/http"
+	"net/http/httptest"
+	"os"
+	"path/filepath"
+	"slices"
+	"strings"
+	"testing"
+
+	"github.com/docker/docker/client"
+)
+
+// File names used in more than one test build context, as constants to
+// satisfy the goconst linter.
+const (
+	testMainGo           = "main.go"
+	testSecretFile       = "secret.txt"
+	testDeployDockerfile = "deploy/Dockerfile"
+)
+
+// TestPerformBuildFollowsDockerignore runs builds against a fake Docker API
+// and checks which files the build context sent to it holds.
+func TestPerformBuildFollowsDockerignore(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name       string
+		dockerfile string
+		files      map[string]string // path in the context: contents
+		want       []string          // files sent in the build context
+	}{
+		{
+			name:       "no ignore file",
+			dockerfile: defaultDockerfileName,
+			files:      map[string]string{defaultDockerfileName: "", testMainGo: ""},
+			want:       []string{defaultDockerfileName, testMainGo},
+		},
+		{
+			name:       "excludes and re-includes",
+			dockerfile: defaultDockerfileName,
+			files: map[string]string{
+				defaultDockerignoreName: "secret.txt\n*.md\n!README.md\n",
+				defaultDockerfileName:   "",
+				"NOTES.md":              "",
+				"README.md":             "",
+				testMainGo:              "",
+				testSecretFile:          "",
+			},
+			want: []string{
+				defaultDockerignoreName, defaultDockerfileName, "README.md", testMainGo,
+			},
+		},
+		{
+			name:       "keeps the Dockerfile and .dockerignore",
+			dockerfile: defaultDockerfileName,
+			files: map[string]string{
+				defaultDockerignoreName: "Dockerfile\n.dockerignore\nsecret.txt\n",
+				defaultDockerfileName:   "",
+				testMainGo:              "",
+				testSecretFile:          "",
+			},
+			want: []string{defaultDockerignoreName, defaultDockerfileName, testMainGo},
+		},
+		{
+			name:       "an ignore file next to the Dockerfile wins over .dockerignore",
+			dockerfile: testDeployDockerfile,
+			files: map[string]string{
+				defaultDockerignoreName:          "main.go\n",
+				testDeployDockerfile:             "",
+				"deploy/Dockerfile.dockerignore": "secret.txt\n",
+				testMainGo:                       "",
+				testSecretFile:                   "",
+			},
+			want: []string{
+				defaultDockerignoreName,
+				testDeployDockerfile,
+				"deploy/Dockerfile.dockerignore",
+				testMainGo,
+			},
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			contextDir := t.TempDir()
+			writeFiles(t, contextDir, tt.files)
+
+			got, err := buildContextFiles(t, contextDir, tt.dockerfile)
+			if err != nil {
+				t.Fatal(err)
+			}
+
+			if !slices.Equal(got, tt.want) {
+				t.Errorf("build context holds %q, want %q", got, tt.want)
+			}
+		})
+	}
+}
+
+// TestPerformBuildFailsOnMalformedDockerignore checks that a pattern the
+// docker command line would reject fails the build.
+func TestPerformBuildFailsOnMalformedDockerignore(t *testing.T) {
+	t.Parallel()
+
+	contextDir := t.TempDir()
+	writeFiles(t, contextDir, map[string]string{defaultDockerignoreName: "[\n"})
+
+	_, err := buildContextFiles(t, contextDir, defaultDockerfileName)
+
+	want := "failed to create build context: " +
+		"invalid pattern in .dockerignore: syntax error in pattern"
+	if err == nil || err.Error() != want {
+		t.Errorf("got error %v, want %q", err, want)
+	}
+}
+
+// TestPerformBuildFailsOnUnreadableDockerignore checks that an ignore file
+// that cannot be read, here because it is a directory, fails the build.
+func TestPerformBuildFailsOnUnreadableDockerignore(t *testing.T) {
+	t.Parallel()
+
+	contextDir := t.TempDir()
+
+	err := os.Mkdir(filepath.Join(contextDir, defaultDockerignoreName), 0o750)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	_, err = buildContextFiles(t, contextDir, defaultDockerfileName)
+
+	want := "failed to create build context: failed to read .dockerignore: "
+	if err == nil || !strings.HasPrefix(err.Error(), want) {
+		t.Errorf("got error %v, want one starting %q", err, want)
+	}
+}
+
+// writeFiles writes files, a map of paths inside dir to their contents,
+// creating the directories they are in.
+func writeFiles(t *testing.T, dir string, files map[string]string) {
+	t.Helper()
+
+	for name, contents := range files {
+		path := filepath.Join(dir, name)
+
+		err := os.MkdirAll(filepath.Dir(path), 0o750)
+		if err != nil {
+			t.Fatal(err)
+		}
+
+		err = os.WriteFile(path, []byte(contents), 0o600)
+		if err != nil {
+			t.Fatal(err)
+		}
+	}
+}
+
+// buildContextFiles runs a build of contextDir against a fake Docker API and
+// returns the names of the files in the build context sent to it, sorted.
+func buildContextFiles(t *testing.T, contextDir, dockerfile string) ([]string, error) {
+	t.Helper()
+
+	sent := make(chan []string, 1)
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			switch {
+			case strings.HasSuffix(r.URL.Path, "/version"):
+				_, _ = w.Write([]byte(`{"Version":"27.3.1","ApiVersion":"1.47"}`))
+			case strings.HasSuffix(r.URL.Path, "/session"):
+				serveSession(t, w, r, make(chan string, 1))
+			case strings.HasSuffix(r.URL.Path, "/build"):
+				sent <- tarFileNames(t, r.Body)
+			default:
+				t.Errorf("unexpected request to %s", r.URL.Path)
+			}
+		},
+	))
+	t.Cleanup(srv.Close)
+
+	dockerAPI, err := client.NewClientWithOpts(
+		client.WithHost("tcp://" + srv.Listener.Addr().String()),
+	)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	c := &Client{docker: dockerAPI, log: slog.Default()}
+
+	_, err = c.performBuild(t.Context(), BuildImageOptions{
+		ContextDir:     contextDir,
+		DockerfilePath: dockerfile,
+	})
+	if err != nil {
+		return nil, err
+	}
+
+	return <-sent, nil
+}
+
+// tarFileNames returns the names of the regular files in the tar read from r,
+// sorted.
+func tarFileNames(t *testing.T, r io.Reader) []string {
+	t.Helper()
+
+	var names []string
+
+	reader := tar.NewReader(r)
+
+	for {
+		header, err := reader.Next()
+		if errors.Is(err, io.EOF) {
+			break
+		}
+
+		if err != nil {
+			t.Errorf("reading the build context: %v", err)
+
+			return nil
+		}
+
+		if header.Typeflag == tar.TypeReg {
+			names = append(names, header.Name)
+		}
+	}
+
+	slices.Sort(names)
+
+	return names
+}

internal/docker/client.go

@@ -25,7 +25,6 @@ import (
 	"github.com/docker/docker/api/types/network"
 	"github.com/docker/docker/api/types/versions"
 	"github.com/docker/docker/client"
-	"github.com/docker/docker/pkg/archive"
 	"github.com/docker/docker/pkg/jsonmessage"
 	"github.com/docker/docker/pkg/stdcopy"
 	"github.com/docker/go-connections/nat"
@@ -660,7 +659,7 @@ func (c *Client) performBuild(
 	}
 
 	// Create tar archive of build context
-	tarArchive, err := archive.TarWithOptions(opts.ContextDir, &archive.TarOptions{})
+	tarArchive, err := tarBuildContext(opts.ContextDir, opts.DockerfilePath)
 	if err != nil {
 		return "", fmt.Errorf("failed to create build context: %w", err)
 	}