Free Dockerfile test files and sample generator

A valid Dockerfile with FROM / RUN / CMD instructions. Generate sample Dockerfiles for testing linters, CI pipelines and container tooling.

Generate a .dockerfile file →

What is a .dockerfile file?

Dockerfile: Container Build Recipe. A Dockerfile is the build recipe for a container image, a base image, then a sequence of instructions. It is executable configuration, so linting and layer analysis are common CI steps.

Why generate a test Dockerfile file?

• Test a Dockerfile linter, parser or build-analysis tool.
• Check how a CI step handles a file with many layers.
• Verify instruction parsing without running a real build.

How a .dockerfile file is identified

A .dockerfile file has no signature at the start. A text format has no identifying leading bytes, so the extension and the reported media type are the only claims about it until a parser actually reads it, which is why validating uploads by extension alone is unsafe.

How the size lands on the exact byte

The format allows comments, so the padding is comment lines, legal, inert, and visible if you open the file.

Negative testing with broken Dockerfile files

• Invalid. Identifiable, but wrong inside, so it fails during parsing.
• Corrupted. The leading bytes are destroyed. Same length as the valid file.
• Incomplete. Truncated part-way, so it is genuinely shorter.
• Empty. Chosen under Content rather than File Condition: valid structure, no data.

Which failure to reach for, and what to assert for each, is covered in how to make a corrupted file for testing. For upload limits and type checks see testing file upload validation, and for this format's encodings, UTF-16 and BOM test files.

Sample Dockerfile content

FROM alpine:3.19
WORKDIR /app
RUN echo "step 0"
CMD ["sh"]

This shows the shape of the file. Your download is generated fresh at the size you choose, so the values will differ.

How to open and verify the file

Lint with `hadolint Dockerfile`, or parse with `docker build --check`.

Media type text/plain · extension .dockerfile · category Config & DevOps

Generate a .dockerfile file → · questions about sizes, privacy or cost are answered on the site FAQ.

Frequently asked questions about Dockerfile files

What identifies a .dockerfile file?

Nothing at the start of the file does. A text format has no identifying leading bytes, so the extension and the reported media type are the only claims about it until a parser actually reads it, which is why validating uploads by extension alone is unsafe.

How can a .dockerfile file be an exact number of bytes?

The format allows comments, so the padding is comment lines, legal, inert, and visible if you open the file.

Can I download several broken Dockerfile files at once?

Yes, select more than one File Condition and you get one ZIP containing a .dockerfile per condition, each named after its condition.

Can the Dockerfile file use UTF-16 or a byte-order mark?

Yes. .dockerfile is a text format, so it can be generated as UTF-8, UTF-8 with a byte-order mark, UTF-16 LE or UTF-16 BE. The body is shortened before re-encoding so the finished file still lands on the exact size you asked for. Note that a UTF-16 file always has an even byte count.

How do I open or verify the Dockerfile file?

Lint with hadolint Dockerfile, or parse with docker build --check.

Related Config & DevOps formats

ENV · INI · TOML · gitignore · CONF · Properties · Makefile · EditorConfig

Browse all file formats →