Add dockerfile

[roipoussiere]

This PR adds a Dockerfile based on Alpine.

A ronn Dockerfile is useful for Continuous Integration tools (such as GitLab CI), to automatically generate a man page during a build process.

This Dockerfile is currently hosted on Docker hub. If this PR is merged, I can transfer ownship to an other Docker hub user or organization.

Usage

I recommend to use this container with stdin and stdout, in this way we don't have to deal with Docker volumes.

Saving ./my_program.1.ronn manpage to ./my_program.1:

docker run --rm -i roipoussiere/ronn --roff --pipe < ./my_program.1.ronn > ./my_program.1

Saving ./my_program.1.ronn html manpage to ./my_program.1.html:

docker run --rm -i roipoussiere/ronn --html --pipe < ./my_program.1.ronn > ./my_program.1.html

Usage in CI

The image entrypoint must be overridden in your CI tool. For instance, if GitLab CI is used, the .gitlab-ci.yml could look like this:

man:
  image:
    name: roipoussiere/ronn
    entrypoint: ["/bin/sh", "-c"]
  script:
    - ronn --roff --pipe ./my_script.1.ronn > ./my_script.1.ronn
    - ronn --html --pipe ./my_script.1.ronn > ./my_script.1.html

[apjanke]

Could you rebase on master? I think that'll fix the Travis CI build failure.

This seems like a good idea, but I have basically no expertise with Docker, so I don't know how to evaluate it. Can you point me at some doco that would get me familiar enough with Docker, Docker Hub, and its interaction with Travis CI so that I could evaluate this?

[roipoussiere]

Here is the docs: https://docs.docker.com/

[apjanke]

Okay, I've got a bit of experience with Docker now, and I've cleaned up the spurious failures in Travis CI for real (I think). Would you mind rebasing this on master again to see if it runs clean now?

[guenhter]

@roipoussiere Do you mind to rebase again?

[guenhter]

In my experience, it is not the best way to use such a generic image without any version information. Maybe it is better to use a more concrete image tag.