optimise docker image size and adds AWS RDS CA
optimise docker image size and adds AWS RDS CA

The build docker image was crazy big (nearly 800MB!!) so this switches
to using a multi-stage build and an alpine container to only include the
relevent executable in the image taking it down to a more respectable
12MB.

ENTRYPOINT should be prefered over CMD alone so you don't have a random
`sh` in the process tree and the target executable is exec'd directly.

We also want to be able to use router with a AWS DocumentDB backend
(mongodb compatible) to do this we need to have the AWS provided CA used
to verify tls connections. So this fetches the CA and places it at
/etc/ssl/certs where go expects to find it.

alphagov/router