Add Dockerfile

[njourdane]

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