Have 2021 HF Docker image based on Oracle Linux 7

Following NXP-32505, though it might not be strictly necessary, let's make the 2021 HF Docker image based on Oracle Linux, as the 2021 Docker image.

This HF image is only used for the Rainforest hotfix environment, when testing a given build before promotion. E.g. for NCO-24536 we need to deploy docker-internal.packages.nuxeo.com/nuxeo/nuxeo-hf:2021.53.3.

Though the interesting part to test is the Nuxeo server itself, not the Docker image:

• It's more consistent to have both Docker images (nuxeo/nuxeo and nuxeo/nuxeo-hf) based on the same OS.
• It is probably better for the Cloud deployment stack to have both images aligned.