use upper directory of former 'rw' session in 'ro' mode

[trz42]

This PR is essentially redoing #550. Original description below. It is also doing a little code polishing replacing TAB with spaces.

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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.

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Because the mounting of directories under /cvmfs is done late in the eessi_container.sh script, this should also work with the changes introduced in #618, e.g., one should be able to run something like

eessi_container.sh --repository software.eessi.io,access=ro --repository dev.eessi.io,access=rw --storage=/tmp

which reports about a temporary location used such as /tmp/eessi.abc123defg and then

eessi_container.sh --repository software.eessi.io,access=ro --repository dev.eessi.io,access=ro --resume /tmp/eessi.abc123defg

[eessi-bot]

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

• architectures: x86_64/generic, x86_64/intel/haswell, x86_64/intel/skylake_avx512, x86_64/amd/zen2, x86_64/amd/zen3, aarch64/generic, aarch64/neoverse_n1, aarch64/neoverse_v1
• repositories: eessi.io-2023.06-compat, eessi-hpc.org-2023.06-software, eessi-hpc.org-2023.06-compat, eessi.io-2023.06-software

[eessi-bot]

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

• architectures: x86_64/amd/zen4
• repositories: eessi.io-2023.06-compat, eessi-hpc.org-2023.06-compat, eessi-hpc.org-2023.06-software, eessi.io-2023.06-software

[casparvl]

Should we print something to stdout here to notify that we have detected an overlay directory from a previous build, and are mounting it read-only? I think that will help with future debugging if we ever have issues.

[casparvl]

Maybe also add an explicit echo here, just so we easily see in which branch we went.

[casparvl]

Once the two explicit echo's are added to make it more transparent into which branch we went, I'd say this is good to go.

#676 (comment) proves that this enables the test step of running in ro mode with the previous overlay-upper mounted as lowerdir (i.e. read-only). It shows that the patchelf we just built in the build step is available in the test step. At the same time, #677 proves that without this PR (#674 ), the patchelf we just built in the build step is not available in the test step.

In summary: this PR does exactly what it says on the label :)