contrib: add a Dockerfile to run the website locally

[darosior]

It was useful to me, so i cleaned it up and PR'd it in case others do not have a full Ruby toolchain installed, but do have Docker.

[darosior]

(This is based on the CI config)

[janb84]

ACK 9f7a431

PR adds dockerfile (based on ci) and edited readme in contrib with instruction so that the bitcoincore.org website can be run locally for easy testing of changes.

This is a valuable addition and makes testing the changes locally easier without installing all the dependencies on the base machine.

• code reviewed ✅
• followed instructions to run it locally ✅

[willcl-ark]

ACK 9f7a431

Dockerfile is nice and tidy, and works well in testing.

[stickies-v]

Concept ACK, but I think incrementally building the site at runtime makes more sense than doing it at build time (see suggestion).

[janb84]

Concept ACK, but I think incrementally building the site at runtime makes more sense than doing it at build time (see suggestion).

Although I like the suggestions, I think this PR container is ment to behave like the "real website" and therefor is an (almost) duplicate of the CI .
(the CI can use an update but that's a good followup)

[darosior]

Thanks @stickies-v for the rewrite review. I took all your suggestions to the Dockerfile, but let aside the suggested Makefile change.