Add Dockerfile and usage information

[sbkg0002]

This PR will add:

• Multi stage dockerfile to build Alpine based container image
• Information for docker based usage

[anordal]

I think it should suffice to copy only the release/shellharden binary.[1]

I like that this achieves a reproducible build, and that the image overhead is actually less than the size of the executable. I have no problem merging this.

[1] https://stackoverflow.com/questions/47528244/which-files-from-the-target-directory-are-actually-required-by-the-executable

[anordal]

I should mention that I also maintain a shellharden package on crates.io, so that this will do the right thing:

cargo install shellharden

This should alleviate some of the need for a docker image. The problem with that package is that I won't be able to include a manpage in it.

[sbkg0002]

Thanks @anordal. I changed the Dockerfile to only contain the binary.

Btw; it's the cargo dependency that made me create the dockerfile.