Reduce size of explorer docker images

The explorer docker images (nuxeo images + explorer package) weighs 2.5G+ more than the base docker image.
Looking at layers, the following command (performed when adding packages) is the one responsible:

When in install-packages.sh

chmod -R g+rwX $NUXEO_HOME

When in Dockerfile (moved from install-packages for "optim" when several installs are done in a row:

RUN chmod -R g+rwX $NUXEO_HOME

--> check if this line is really needed after installing packages, maybe remove it completely in the explorer use cases (build and export)

Note: this seems to be needed when using Kaniko: this ensures being able to run the docker image as a non-root user.

Symptoms on 10.10 build (using a 10.10 hotfixed image): push of the docker image fails with an error from nexus (size limit issue – did not keep the output), nxginx proxy shows:

2021/04/15 10:28:52 [error] 53951#53951: *7147536 client intended to send too large chunked body: 3221226024 bytes, client: 10.142.0.54, server: docker.XXX.nuxeo.com, request: "PATCH /v2/nuxeo/nuxeo-explorer/blobs/uploads/3d6ec89f-0cb9-4513-9e8b-6679dbbc9a15 HTTP/2.0", host: "docker.XXX.nuxeo.com"