Add EXPORT_ENV_FILE parameter

This allows environment variables from the Dockerfile and
devcontainer.json to be exported to a file that can be sourced later.

envbuilder.go

@@ -194,6 +194,11 @@ type Options struct {
 	// This is useful for self-signed certificates.
 	SSLCertBase64 string `env:"SSL_CERT_BASE64"`
 
+	// ExportEnvFile is an optional file path where envbuilder
+	// will dump environment variables from devcontainer.json
+	// and the built container image.
+	ExportEnvFile string `env:"EXPORT_ENV_FILE"`
+
 	// Logger is the logger to use for all operations.
 	Logger func(level codersdk.LogLevel, format string, args ...interface{})
 
@@ -524,6 +529,7 @@ func Run(ctx context.Context, options Options) error {
 		})
 	}
 
+	skippedRebuild := false
 	build := func() (v1.Image, error) {
 		_, err := options.Filesystem.Stat(MagicFile)
 		if err == nil && options.SkipRebuild {
@@ -537,6 +543,7 @@ func Run(ctx context.Context, options Options) error {
 				return nil, fmt.Errorf("image from remote: %w", err)
 			}
 			endStage("🏗️ Found image from remote!")
+			skippedRebuild = true
 			return image, nil
 		}
 
@@ -668,6 +675,20 @@ func Run(ctx context.Context, options Options) error {
 	}
 	_ = file.Close()
 
+	var exportEnvFile *os.File
+	if options.ExportEnvFile != "" && !skippedRebuild {
+		exportEnvFile, err = os.Create(options.ExportEnvFile)
+		if err != nil {
+			return fmt.Errorf("failed to open EXPORT_ENV_FILE %q: %w", options.ExportEnvFile, err)
+		}
+	}
+	exportEnv := func(key, value string) {
+		if exportEnvFile == nil {
+			return
+		}
+		fmt.Fprintf(exportEnvFile, "export %s=%q\n", key, value)
+	}
+
 	configFile, err := image.ConfigFile()
 	if err != nil {
 		return fmt.Errorf("get image config: %w", err)
@@ -695,6 +716,7 @@ func Run(ctx context.Context, options Options) error {
 			if container.RemoteEnv != nil {
 				for key, value := range container.RemoteEnv {
 					os.Setenv(key, value)
+					exportEnv(key, value)
 				}
 			}
 		}
@@ -724,10 +746,16 @@ func Run(ctx context.Context, options Options) error {
 	for _, env := range configFile.Config.Env {
 		pair := strings.SplitN(env, "=", 2)
 		os.Setenv(pair[0], pair[1])
+		exportEnv(pair[0], pair[1])
 	}
 	for _, env := range buildParams.Env {
 		pair := strings.SplitN(env, "=", 2)
 		os.Setenv(pair[0], pair[1])
+		exportEnv(pair[0], pair[1])
+	}
+
+	if exportEnvFile != nil {
+		exportEnvFile.Close()
 	}
 
 	username := configFile.Config.User

integration/integration_test.go

@@ -353,6 +353,39 @@ func TestExitBuildOnFailure(t *testing.T) {
 	require.ErrorContains(t, err, "parsing dockerfile")
 }
 
+func TestExportEnvFile(t *testing.T) {
+	t.Parallel()
+
+	// Ensures that a Git repository with a devcontainer.json is cloned and built.
+	url := createGitServer(t, gitServerOptions{
+		files: map[string]string{
+			".devcontainer/devcontainer.json": `{
+				"name": "Test",
+				"build": {
+					"dockerfile": "Dockerfile"
+				},
+				"build": {
+					"dockerfile": "Dockerfile"
+				},
+				"remoteEnv": {
+					"FROM_DEVCONTAINER_JSON": "bar"
+				}
+			}`,
+			".devcontainer/Dockerfile": "FROM alpine:latest\nENV FROM_DOCKERFILE=foo",
+		},
+	})
+	ctr, err := runEnvbuilder(t, options{env: []string{
+		"GIT_URL=" + url,
+		"EXPORT_ENV_FILE=/env",
+	}})
+	require.NoError(t, err)
+
+	output := execContainer(t, ctr, "cat /env")
+	require.Contains(t, strings.TrimSpace(output),
+		`export FROM_DOCKERFILE="foo"
+export FROM_DEVCONTAINER_JSON="bar"`)
+}
+
 func TestPrivateRegistry(t *testing.T) {
 	t.Parallel()
 	t.Run("NoAuth", func(t *testing.T) {