fix: concurrent build issue, closes #2782

[a-h]

Closes #2782

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[CLAassistant]

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[PE39806]

Thank you for both identifying and fixing this bug. Once you have signed our CLA, please do merge this branch in.

[GCHQDeveloper36]

I added a fix for the other images in #2238 by adding sharing=locked to the mount options (see https://github.com/gchq/Bailo/pull/2238/files#diff-fed51f49a9f26cb93cc870efdc9419d425b9422354ae41bb651c3333c8bff486R10 for an example) - could we do the same here instead? Otherwise the caches aren't reused by different commands.

[PE39806]

See #2783 (comment)