Set an explicit UID and GID for the opal user

[chgl]

Currently, the adduser command tends to create a user with an arbitrary UID, depending on the base image (https://github.com/obiba/docker-opal/blob/main/Dockerfile#L66). This makes it difficult to run opal in Kubernetes where explicitly setting the user id and group ip as part of the container's security context is necessary. I noticed this being a problem as both the docker.io/obiba/opal:5.0.2 and the docker.io/datashield/rock-base:6.3.1 tend to change the current UID between updates, causing updates to fail.

It would be great if the opal user's user and group id could be set to a fixed value and a USER directive be added to the Dockerfile. I'd be happy to create a PR if this seems reasonable!

[ymarcon]

PRs are more than welcomed!

FYI I recently tried to replace gosu by the USER directive but had some issues with access rights on the mounted volume.