Dockerfile: Move to project root and combine RUNs

Dockerfile: Move to project root and combine RUNs

According to https://docs.docker.com/develop/develop-images/dockerfile_best-practices/#run
`apt-get update` should always be combined with `apt-get install` in
the same `RUN` statement, to avoid installation of outdated packages.

Also, combine some more `RUN`s together as to avoid fewer layers.

Finally, remove some comments which are superflous. Comments shouldn't
describe what is done, especially when it's obvious. They should
explain why something is done.

Signed-off-by: Christian Franke <chris@opensourcerouting.org>