Hello with the help of @diana_harosa I was able to make things work inside the docker container
To be able to understand if this is a SNAP issue or a server-side issue.
In the <SNAP_INSTALL_FOLDER>/etc/snap.auxdata.properties file, could you replace the following line:
DEM.srtm3GeoTiffDEM_HTTP = https://download.esa.int/step/auxdata/dem/SRTM90/tiff/
with
DEM.srtm3GeoTiffDEM_HTTP = https://step.esa.int/auxdata/dem/SRTM90/tiff/ ?
This is what has been suggested as possible solution (which worked for my issue) and inside the docker container I added the following line after the SNAP update (as final command):
RUN sed -i 's/https:\/\/download.esa.int\/step\/auxdata\/dem\/SRTM90\/tiff\//https:\/\/step.esa.int\/auxdata\/dem\/SRTM90\/tiff\//g' /usr/local/snap/etc/snap.auxdata.properties