[Feat] Explain docker image usage

[neteler]

We provide a set of Docker images at https://grass.osgeo.org/download/docker/

Especially for the image with GUI support, it can be difficult to get started (see related messages on the grass-user mailing list).

So let's add some usage instructions to lower the barrier:

# GRASS GIS docker image with GUI support
docker pull osgeo/grass-gis:releasebranch_8_4-ubuntu_wxgui

docker run -it --rm --volume="$(pwd)/:/data" \
       --volume="/tmp/.X11-unix:/tmp/.X11-unix:rw" \
       --env DISPLAY=$DISPLAY \
       --device="/dev/dri/card0:/dev/dri/card0" \
       osgeo/grass-gis:releasebranch_8_4-ubuntu_wxgui \
       grass --gui

Possibly this could be an expandable info button or something like that to not clutter the page.

[wenzeslaus]

What about a link to a readme file in a directory with Dockerfiles which at least somewhat serves as a doc for our Docker images?

[neteler]

So I will expand
https://github.com/OSGeo/grass/blob/main/docker/README.md

Once merged it may be added to the docker download page.

[cwhite911]

@neteler I included the existing text from the GRASS README in PR #427, but will switch it once it is merged.