Add the datahub plugin docker image + Build docker image on github

[Guillaume-d-o]

This PR introduces two main changes:

• Adds a Dockerfile to build the geonetwork-datahub Docker image
• Adds a GitHub workflow to build the images (geonetwork and geonetwork-datahub) automatically

[jahow]

Here's a first review to hopefully make the image simpler! thanks

[jahow]

the base image already has unzip and uses curl for the download so I guess this install step is not necessary?

[jahow]

and switching users as well

[jahow]

this variable is probably not necessary

[jahow]

this comment can be safely removed

[jahow]

see base image for how to download the file using curl; an MD5 check step should also be included

[Guillaume-d-o]

as the GN documentation give this url (for the GN war) without checking is it necessary ? cf https://docs.geonetwork-opensource.org/3.12/fr/install-guide/installing-from-war-file/

[jahow]

I would argue it is necessary, and also there's not much reason not to do it :)

[Guillaume-d-o]

Done :)

[jahow]

The base image uses GN_VERSION so defining this new variable is probably not needed

[juanluisrp]

For the official images we can't build or publish the images using this repository. This is done by the Docker Official images team using the contents of this repository.

We can however build the datahub image and publish to the GeoNetwork Docker Hub organisation at https://hub.docker.com/u/geonetwork.

If we want an official Docker image for geonetwork-datahub the best option would be to create a folder in the specific release directory like we did for the -postgres variant of the image in the 3.12.x series (https://github.com/geonetwork/docker-geonetwork/tree/main/3.12.9/postgres) with the specific Dockerfile for geonetwork-datahub there. Then we would need that variant to the https://github.com/geonetwork/docker-geonetwork/blob/main/generate-stackbrew-library.sh.