docker: Document OPENGROK_WEBAPP_CONTEXT environment variable

Author: timschumi

docker/README.md

@@ -75,6 +75,7 @@ The volume mounted to `/opengrok/src` should contain the projects you want to ma
 `REINDEX: <time_in_minutes>`<br/> *Optional* *Default: 10* | Period of automatic mirroring/reindexing. Setting to `0` will disable automatic indexing. You can manually trigger an reindex using docker exec: `docker exec <container> /scripts/index.sh`
 `INDEXER_OPT` | pass extra options to opengrok-indexer. For example, "-i d:vendor" will remove all the `*/vendor/*` files from the index. You can check the indexer options on https://github.com/oracle/opengrok/wiki/Python-scripts-transition-guide
 `NOMIRROR` | To avoid the mirroring step, set the variable to non-empty value.
+`OPENGROK_WEBAPP_CONTEXT`<br/> *Default: /* | Override the sub-URL that OpenGrok should run on.
 
 To specify environment variable for `docker run`, use the `-e` option, e.g. `-e REINDEX=30`