Add support for context path

[ckullabosch]

I wanted to run a Gerrit with a context path, like https://<hostname>/gerrit instead of https://<hostname>. Therefore I added these entries to the gerrit.config:

[httpd]
listenUrl = proxy-http://*:8080/gerrit
[gerrit]
canonicalWebUrl = https://<hostname>/gerrit/

This didn't worked out, the container was never ready and restarted endless. The health probes invoke /config/server/healthcheck~status and not <context-path>/config/server/healthcheck~status and therefore always fail.

It would be great to have support for configuring the contextPath.