Changes to allow building a devkit using podman on a RHEL8 host

[sxa]

Both of these changes are based on equivalents in the build pipelines:

• Adds UID option for podman to ensure bind mounts are writable by the jenkins user
• Avoid docker pull command if the image name contains rhel

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[karianna]

@sxa rebase required.

[sxa]

@sxa rebase required.

Done - that was a tricky one, but I always knew it was going to be as I'd based it off a branch for one of Andrew's PRs which has now been merged.

[andrew-m-leonard]

jdk21u probably ought to be ${VERSION}

[andrew-m-leonard]

I actually think you are already in build/devkit/jdk21u
so you just need:

mkdir -p build/devkit/download/rpms/s390x-linux-gnu-Centos${BASE_OS_VERSION}

[sxa]

As discussed yesterday I'll leave that for now and fix it in the future - I don't want to change anything else now that it's been fully tested given we want to rely on this for the release in a couple of week's time.