enable mounting previous overlay-upper read-only

[trz42]

Use case for this is that we want to test software additions to a CernVM-FS repository. Typically we would first run eessi_container.sh with --access rw to make the repository writable with an overlayfs. The changes are stored in some overlay-upper directory (under some /tmp dir or in a tarball of that /tmp directory). When we want to test the additions later we can run eessi_container.sh with --resume TMPDIR_OR_TMPTARBALL. However, until now this only worked when using --access rw which would make the repository again writable, which is likely not what is wanted in a test (because in actual use /cvmfs/* mounts are read-only). In fact, the test-suite.sh run after a build job has finished, uses --access ro (read-only), but this access mode did not recognise changes in an overlay-upper directory.

This PR merges an existing overlay-upper (given via --resume ...) on top of a CernVM-FS repository. Because it only uses the lowerdir option of fuse-overlayfs the merged directory is mounted read-only.

[eessi-bot-aws[bot]]

Instance eessi-bot-mc-aws is configured to build:

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• arch x86_64/generic for repo eessi-hpc.org-2023.06-software
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• arch aarch64/generic for repo eessi-hpc.org-2023.06-software
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• arch aarch64/neoverse_v1 for repo eessi-hpc.org-2023.06-software
• arch aarch64/neoverse_v1 for repo eessi.io-2023.06-compat
• arch aarch64/neoverse_v1 for repo eessi.io-2023.06-software

[eessi-bot-aws[bot]]

Instance eessi-bot-mc-azure is configured to build:

• arch x86_64/amd/zen4 for repo eessi-hpc.org-2023.06-compat
• arch x86_64/amd/zen4 for repo eessi-hpc.org-2023.06-software
• arch x86_64/amd/zen4 for repo eessi.io-2023.06-compat
• arch x86_64/amd/zen4 for repo eessi.io-2023.06-software