Use sqlite by default in docker

This allows for easy setup with no extra configuration.
Also update docs to reflect the new and easy way to run PowerDNS-Admin.

Signed-off-by: Felix Kaechele <felix@kaechele.ca>

docker/entrypoint.sh

@@ -11,7 +11,9 @@ cat ./powerdnsadmin/default_config.py ./configs/docker_config.py > ./powerdnsadm
 
 GUNICORN_ARGS="-t ${GUNICORN_TIMEOUT} --workers ${GUNICORN_WORKERS} --bind ${BIND_ADDRESS} --log-level ${GUNICORN_LOGLEVEL}"
 if [ "$1" == gunicorn ]; then
-    flask db upgrade
+    # run as user pda so that if a SQLite database is generated it is writeable
+    # by that user
+    su pda -s /bin/sh -c "flask db upgrade"
     exec "$@" $GUNICORN_ARGS
 
 else