Make image smaller

[chrisburr]

Currently the image is twice the size due to the chown $MAMBAUSER. It should be collapsed down to a single layer (maybe setfacl can help?).

[chaen]

Document that we can interact with the demo like that
https://github.com/DIRACGrid/diracx-charts/blob/bc3baa0a1250abe9ee8da5b9103d16b39689d4d2/run_demo.sh#L461