Add Dockerfile

[carlosms]

Fix #5.

This PR adds the Dockerfile. The entrypoint does not run any sub command by default, because sync will fail without calling migrate first. Both are needed, so I though it makes sense to leave ghsync and it will print the help by default.

$ docker run srcd/ghsync
Please specify one command of: completion, migrate, sync or version
Usage:
...

ca-certificates added for the https github endpoints.

I left a sleep in docker-compose.yml to wait for postgres & rabbitmq initialization. This can be improved in a couple of ways:

• make ghsync actively retry the connection. Maybe every 5s for 2m.
• make the docker entrypoint a wrapper that pings and waits for postgres & rabbitmq.

I didn't want to complicate it now, as this can be reconsidered when we have a better understanding of how this will be integrated in sourced-ce.

[carlosms]

@dpordomingo please take another look.