Build apps with BuildKit so build stages stop piling up (closes #220)

Multi-stage app builds left an untagged image for every build stage after each deploy, and nothing could safely remove them. upaas now asks Docker for a BuildKit build, which keeps stages in Docker's build cache instead of as images; Docker limits and cleans that cache itself. The final image is still tagged `upaas-<app>:<N>`, so old-image cleanup is unchanged. BuildKit's progress messages are decoded and written to the deployment log as plain text.

Deviation: adds `github.com/moby/buildkit` v0.16.0 (matching the pinned Docker client), added with `go get` since no make target adds dependencies.
Judgement call: the cache limit is on by default only from Docker Engine 28.2; older engines need the `daemon.json` setting the README now names.

Model: opus-5-5

internal/docker/validation_test.go

@@ -1,7 +1,9 @@
 package docker //nolint:testpackage // tests unexported regexps and Client struct
 
 import (
+	"bytes"
 	"context"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"log/slog"
@@ -11,8 +13,10 @@ import (
 	"path/filepath"
 	"strings"
 	"testing"
+	"time"
 
 	"github.com/docker/docker/client"
+	controlapi "github.com/moby/buildkit/api/services/control"
 )
 
 // mainBranch is the branch name used across validation tests.
@@ -239,3 +243,74 @@ func TestPerformCloneRemovesContainerVolumes(t *testing.T) {
 		})
 	}
 }
+
+// TestPerformBuildUsesBuildKit runs a build against a fake Docker API and
+// checks that it asks for BuildKit and that BuildKit's progress reaches the
+// build log as plain text.
+func TestPerformBuildUsesBuildKit(t *testing.T) {
+	t.Parallel()
+
+	now := time.Now()
+	status := controlapi.StatusResponse{Vertexes: []*controlapi.Vertex{{
+		Digest:    "sha256:1111",
+		Name:      "[build 2/2] RUN make",
+		Started:   &now,
+		Completed: &now,
+	}}}
+
+	data, err := status.Marshal()
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	trace, err := json.Marshal(data)
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	srv := httptest.NewServer(http.HandlerFunc(
+		func(w http.ResponseWriter, r *http.Request) {
+			switch {
+			case strings.HasSuffix(r.URL.Path, "/build"):
+				if r.URL.Query().Get("version") != "2" {
+					http.Error(w, "not a BuildKit build", http.StatusBadRequest)
+
+					return
+				}
+
+				_, _ = fmt.Fprintf(w, "{\"id\":\"moby.buildkit.trace\",\"aux\":%s}\n", trace)
+			default:
+				_, _ = w.Write([]byte(`{"Id":"sha256:built"}`))
+			}
+		},
+	))
+	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()}
+
+	var buildLog bytes.Buffer
+
+	imageID, err := c.performBuild(t.Context(), BuildImageOptions{
+		ContextDir: t.TempDir(),
+		Tags:       []string{"upaas-test:1"},
+		LogWriter:  &buildLog,
+	})
+	if err != nil {
+		t.Fatal(err)
+	}
+
+	if imageID != "sha256:built" {
+		t.Errorf("unexpected image ID %q", imageID)
+	}
+
+	if !strings.Contains(buildLog.String(), "[build 2/2] RUN make") {
+		t.Errorf("build log is missing the build step:\n%s", buildLog.String())
+	}
+}