Run phoebus when the container is started

I wanted to be able to run the container and immediately have phoebus start without having to attach and run /opt/phoebus/phoebus.sh or run a seperate exec command.

I looked at the example from pklaus https://github.com/pklaus/docker-phoebus and added an entrypoint at the end of the Dockerfile

I also modified the README.md to include instructions that do not require any configuration from the user for the DISPLAY parameter from their host environment. Instead I read this environment variable using $DISPLAY which is something we also do with the bluesky_container.

Additionally I use the argument --rm in the docker run command which deletes the container as soon as the application is closed by the host. This avoids having many many containers in the history. I don't know if this is important, but it looks cleaner.

Finally I wrap all of this in a bash script as an example which could be added to a bash.rc file