Dockerize without elixir releases

[amberrignell]

• Install dependencies
• Set mix phx.server as entrypoint
• Ensure static assets are served to browser
• Provide instructions on README
• Provide docker-compose example (for Postgres)

Resources

• https://github.com/nickjj/docker-phoenix-example - big and beefy
• https://gist.github.com/petelacey/b8a1aacdc33c8718ba9366733e16a8c2
• https://github.com/dogweather/phoenix-docker-compose

[amberrignell]

Hey team! Please add your planning poker estimate with ZenHub @elias-ba @stuartc @taylordowns2000

[stuartc]

@taylordowns2000 these are great resources, especially the first one -
I really like it, very comprehensive; but as your 'big and beefy' option accurately relays how big it is... I'll be pinching bits of it's dockerfile and docker-compose files (the way we use tailwind I think also removes the requirement to step into assets for a build)...
And maybe I'll use some of the run shell script for a 'bootstrap'.. so basically I'm going to use the first example but take just the basics for now 👍

[stuartc]

@taylordowns2000 this is ready for review when you have a moment over here: #67

[stuartc]

@taylordowns2000 ok this looks good to go...

@amberrignell here is a copy of the README for this pr