Add docker image to the repo

[Fokko]

I think it makes sense to have the Dockerfile in the repository itself.

Resolves #739

resolves # docs dbt-labs/docs.getdbt.com/#

Problem

Solution

Checklist

• [ ] I have read the contributing guide and understand what's expected of me
• [ ] I have run this code in development and it appears to resolve the stated issue
• [ ] This PR includes tests, or tests are not required/relevant for this PR
• [ ] This PR has no interface changes (e.g. macros, cli, logs, json artifacts, config files, adapter interface, etc) or this PR has already received feedback and approval from Product or DX

[Fokko]

cc @JCZuurmond

[Fokko]

cc @emmyoop Sorry for the long wait!