Automatic container image build

[leonardehrenfried]

Summary

This integrates a docker image build and push to Dockerhub to the CI pipeline.

Until I have figured out access to the opentripplanner org on Dockerhub I'm deploying to my personal account. I'm working with @abyrd to figure it out.

If you want to test it already, you can use the tutorial.

# create directory for data and config
mkdir berlin
# download OSM
curl -L https://download.geofabrik.de/europe/germany/berlin-latest.osm.pbf -o berlin/osm.pbf  
# download GTFS
curl -L https://vbb.de/vbbgtfs -o berlin/vbb-gtfs.zip
# build graph and save it onto the host system via the volume
docker run --rm -v ./berlin:/var/opentripplanner docker.io/opentripplanner/opentripplanner:latest --build --save
# load and serve graph
docker run -it --rm -p 8080:8080 -v ./berlin:/var/opentripplanner docker.io/opentripplanner/opentripplanner:latest --load --serve

@optionsome @tvainika @jannefleischer Do you want to try this out?

Closes #3291

Documentation

Page added.

[leonardehrenfried]

Is you can see on https://hub.docker.com/r/opentripplanner/opentripplanner/tags I have received the correct permissions for the user otpbot to push to Dockerhub.

This is now ready for review.