What causes "building from STDIN is not supported" in docker/compose?

Set DOCKER_BUILDKIT=1 so the BuildKit path handles stdin contexts

building from STDIN is not supported

Error message

building from STDIN is not supported

What it means

doBuildClassic calls build.DetectContextType on the resolved build context; when the context string is `-` (ContextTypeStdin) the classic builder cannot proceed because this code path builds a tar archive from a directory and has no way to consume a Dockerfile/context piped on stdin. Only BuildKit supports stdin contexts (it base64-encodes the Dockerfile into the API request).

Source

Thrown at pkg/compose/build_classic.go:166

	if service.Build.Labels == nil {
		service.Build.Labels = make(map[string]string)
	}
	service.Build.Labels[api.ImageBuilderLabel] = "classic"

	dockerfileName := dockerFilePath(service.Build.Context, service.Build.Dockerfile)
	specifiedContext := service.Build.Context
	progBuff := s.stdout()
	buildBuff := s.stdout()

	contextType, err := build.DetectContextType(specifiedContext)
	if err != nil {
		return "", err
	}

	switch contextType {
	case build.ContextTypeStdin:
		return "", fmt.Errorf("building from STDIN is not supported")
	case build.ContextTypeLocal:
		contextDir, relDockerfile, err = build.GetContextFromLocalDir(specifiedContext, dockerfileName)
		if err != nil {
			return "", fmt.Errorf("unable to prepare context: %w", err)
		}
		if strings.HasPrefix(relDockerfile, ".."+string(filepath.Separator)) {
			// Dockerfile is outside build-context; read the Dockerfile and pass it as dockerfileCtx
			dockerfileCtx, err = os.Open(dockerfileName)
			if err != nil {
				return "", fmt.Errorf("unable to open Dockerfile: %w", err)
			}
			defer dockerfileCtx.Close() //nolint:errcheck
		}
	case build.ContextTypeGit:
		var tempDir string
		tempDir, relDockerfile, err = build.GetContextFromGitURL(specifiedContext, dockerfileName)
		if err != nil {
			return "", fmt.Errorf("unable to prepare context: %w", err)

View on GitHub (pinned to ddc4b044b6)

Solutions

Set DOCKER_BUILDKIT=1 so the BuildKit path handles stdin contexts
Prefer a concrete context directory in the Compose file (e.g. `context: .`) instead of '-'
Check the interpolated value of the context variable — an empty var defaulting to '-' is usually a configuration mistake

Example fix

# before
services:
  app:
    build:
      context: "${BUILD_CONTEXT:--}"

# after
services:
  app:
    build:
      context: "${BUILD_CONTEXT:-.}"

Defensive patterns

Strategy: validation

Validate before calling

ctx := strings.TrimSpace(svc.Build.Context)
if ctx == "-" {
    return fmt.Errorf("service %s: stdin context requires DOCKER_BUILDKIT=1", svc.Name)
}

Prevention

Avoid '-' as a context in Compose files; use a real directory
Set a safe interpolation default like ${BUILD_CONTEXT:-.} instead of ${BUILD_CONTEXT:--}

When it happens

Trigger: A Compose file (or interpolated value) that sets the service build context to `-`, e.g. `context: -`, combined with the classic builder path; commonly from `docker compose build -f-` style stdin usage or templated context fields resolving to '-'.

Common situations: Piping generated Compose files via stdin with env interpolation making the context '-'; scripts that reuse `docker build -` idioms inside Compose; YAML interpolation bugs where a variable holding the context is empty and a fallback '-' is used.

Related errors

AI-assisted analysis of docker/compose@ddc4b044b6 (2026-08-15). Data as JSON: /api/errors/a5e329a50ac082a6. Report an issue: GitHub.