Add Dockerized build environment & scripts

[ryanprior]

This PR contains

• bin/build-container, a script which builds a Jekyll container using the local Gemfile
• Dockerfile, which provides further build instructions to Docker
• bin/serve, a script which serves the site using the built container
• updates to the README explaining how to use the above

Motivation

By managing dependencies in containers, I avoid package version conflicts on my machine and I can present easily reproducible build and test instructions to others.

Dependencies

This PR adds an optional dependency on Docker.

How to test

Notice that there's a new sub-header under "Building" in the README. Check out this branch and follow the instructions there to build a container and serve the site. This should succeed with no errors.

I tested this with Docker version 19.03.8, build afacb8b7f0

[jeremypw]

@cassidyjames Is this of interest? I cannot do anything with this PR.

[RMcNeely]

There's an official docker image that I think would be better used if this is something we want to pursue.

[RMcNeely]

That said it works well! 🎉

[ryanprior]

@RMcNeely which official docker image do you mean? Happy to update this PR if I can make it more useful.

[RMcNeely]

This is the one referenced in the Jekyll docs: https://hub.docker.com/u/jekyll
Personally I like the idea of Docker files for the web sites but I would really want it for the main website so I don't have to manage PHP. RVM is perfectly usable but Dockerfiles are nice

[ryanprior]

@RMcNeely updated this PR to use your recommended base image & re-verified that it works with the provided script setup.

Happy to work on giving the main website the same treatment, should I open an issue in https://github.com/elementary/website/?

[RMcNeely]

Well elementary/website is the one thing that is not actually based on Jekyll so that one would be more involved (needs both NodeJS and PHP Composer) but I do think it's worth doing. The other Jekyll based sites should be similar to this.

[ryanprior]

Is there a list of all the Jekyll-based websites? I can probably do those similar to this one, and work on a separate custom container image for the main website.

[RMcNeely]

Not that I'm aware of. Off the top of my head it's appcenter-web, releases, blog and technically edw but there may be others.

[kgrubb]

There's some very minor conflicts in the README.md that need to be handled before merging, but aside from that, this looks good to me!