Docker "the Dockerfile (Dockerfile) cannot be empty" in CI

Docker received a Dockerfile with no content. The file exists and is found, but it has zero usable instructions - usually because it is empty, gitignored, or was never written.

What this error means

A docker build fails right away with the Dockerfile (Dockerfile) cannot be empty. Unlike "failed to read dockerfile", the file is located - it just has nothing in it.

the Dockerfile (Dockerfile) cannot be empty
# or, when building from stdin with no input:
docker build - < /dev/null   ->   Dockerfile cannot be empty

Common causes

The Dockerfile is genuinely empty

A zero-byte or whitespace-only file (a placeholder that was never filled in, or truncated by a bad write) has no instructions for Docker to build.

Building from stdin with no piped content

A docker build - reading the Dockerfile from standard input gets nothing when the upstream command produced no output, so Docker sees an empty file.

The file was gitignored or excluded from checkout

A .gitignore or sparse checkout can leave a placeholder/empty Dockerfile on the runner even though a real one exists on a developer machine.

How to fix it

Confirm the file has real content on the runner

Check the size and first lines from the job before building.

wc -c Dockerfile          # 0 bytes means empty
head -5 Dockerfile        # should start with a FROM (or # syntax=)

Ensure stdin builds actually receive a Dockerfile

When piping, verify the producing command emits the Dockerfile content.

# pipe a real Dockerfile, not empty input
cat Dockerfile | docker build -t api -f - .

How to prevent it

• Never commit empty placeholder Dockerfiles; add at least a FROM.
• Make sure the Dockerfile is tracked in git and not ignored or sparse-excluded.
• For stdin builds, assert the piped content is non-empty before building.

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