Upgrade Geoserver image

[giohappy]

GeoNode/geoserver-docker#69

[giohappy]

@randomorder as mentioned in the upstream issue reported in the description, there's a request to upgrade the Tomcat version for the Geoserver docker image used by geonode and geonode-project.

GeoNode uses the image built from geoserver-docker.
geonode-project uses the local definition.

We need to check if un upgrade is really useful in terms of security vulnerabilities and, if yes, which version fits well wtih GS 2.17.4 for GeoNode.