[Dockerfile] Combine apt cmds, clean up after installation
[Dockerfile] Combine apt cmds, clean up after installation

Combine `apt-get update` and `apt-get install` to prevent docker to
create additional image layers, and then clean up apt cache in the same
"RUN" action to make the images smaller

gratipay/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com,gratipay/gratipay.com