Adding Docker container and updating README

[awiens]

This container can be built and run locally, in order to provide a simple way for users to run Gas on their code. The README has been updated to give information and provide build/run commands.

[gcmurphy]

This seems fine for the current branch. Will need to revisit this when the 'experimental' branch is finally merged.

Couple of nitpicks -

1. Can the line spacing be updated so there is one clear grouping of the 'builder' and another 'gas' tool.

Something like:

# Builds the thing
FROM ...
ENV .. 
COPY .. 
WORKDIR ... 
RUN ...

# Runs the thing
FROM ..
LABEL ..
COPY .. 
CMD ..

2. We might also need to set the GOPATH environment variable either via the instructions or setting it in the Dockerfile.

3. I'm not sure if you'll need to set a maintainer if this isn't something going to be published on dockerhub?

[awiens]

I think we are intending to publish to dockerhub at some point, but wanted to just get it up on github first