Add docker-compose.yml for deploying upaas (closes #223)

The compose file builds the image from this repo, mounts the Docker
socket and HOST_DATA_DIR (passed to upaas as UPAAS_HOST_DATA_DIR),
reads settings from .env, and restarts unless stopped. The port is
published on 127.0.0.1 only, for a TLS-terminating proxy in front.
PORT and UPAAS_DATA_DIR are pinned so .env cannot move upaas off the
port mapping, the healthcheck (busybox wget) or the data mount.

upaas now refuses to start when UPAAS_HOST_DATA_DIR is set to a
relative path; when unset it still falls back to the data directory.

The README's plain-HTTP Compose example becomes a short deploy section
that points at the file. .env is added to .dockerignore.

Model: opus-5-5

internal/config/config.go

@@ -32,6 +32,12 @@ const (
 	filePermissions = 0o600
 )
 
+// errHostDataDirNotAbsolute is returned when UPAAS_HOST_DATA_DIR is set to a
+// relative path, which the Docker daemon cannot resolve for app bind mounts.
+var errHostDataDirNotAbsolute = errors.New(
+	"UPAAS_HOST_DATA_DIR must be an absolute path",
+)
+
 // Params contains dependencies for Config.
 type Params struct {
 	fx.In
@@ -124,6 +130,10 @@ func buildConfig(log *slog.Logger, params *Params) (*Config, error) {
 	dataDir := viper.GetString("DATA_DIR")
 	hostDataDir := viper.GetString("HOST_DATA_DIR")
 
+	if hostDataDir != "" && !filepath.IsAbs(hostDataDir) {
+		return nil, fmt.Errorf("%w, got %q", errHostDataDirNotAbsolute, hostDataDir)
+	}
+
 	if hostDataDir == "" {
 		hostDataDir = dataDir
 	}

internal/config/config_test.go

@@ -0,0 +1,33 @@
+package config //nolint:testpackage // tests unexported buildConfig
+
+import (
+	"errors"
+	"log/slog"
+	"testing"
+)
+
+func TestBuildConfigRejectsRelativeHostDataDir(t *testing.T) {
+	t.Setenv("UPAAS_HOST_DATA_DIR", "./data")
+	setupViper("upaas")
+
+	_, err := buildConfig(slog.Default(), &Params{})
+	if !errors.Is(err, errHostDataDirNotAbsolute) {
+		t.Fatalf("expected errHostDataDirNotAbsolute, got %v", err)
+	}
+}
+
+func TestBuildConfigHostDataDirDefaultsToDataDir(t *testing.T) {
+	t.Setenv("UPAAS_DATA_DIR", "./data")
+	t.Setenv("UPAAS_HOST_DATA_DIR", "")
+	t.Setenv("UPAAS_SESSION_SECRET", "test-secret")
+	setupViper("upaas")
+
+	cfg, err := buildConfig(slog.Default(), &Params{})
+	if err != nil {
+		t.Fatalf("unexpected error: %v", err)
+	}
+
+	if cfg.HostDataDir != "./data" {
+		t.Errorf("expected HostDataDir ./data, got %q", cfg.HostDataDir)
+	}
+}