Dockerfile and default DB for user DB management
init, update and start the user_db container if needed
updated database path to user_db volume
changed default port to '80', use 'start-webrob-5000' script if you already run a webserver at 80
added start-webrob-5000 script to start the web interface at port 5000
modified startup scripts: only one script, configurable exposed port, defaults to 80, start with 'KNOWROB_WEB_PORT=5000 start-webrob' to set another port