docker: change base image to almalinux

[ntarocco]

• changes Python version to 3.9, which is the version installed in Almalinux:1
• closes https://github.com/inveniosoftware/cookiecutter-invenio-rdm/issues/238

[ntarocco]

Discussion on-going on the InvenioRDM Discord forum. Waiting a few days before going ahead and merge.