CI: upload docker cache only from main branch

[bayandin]

Problem

The Docker build cache got invalidated by PRs

Summary of changes

• Upload cache only from the main branch

Checklist before requesting a review

• [x] I have performed a self-review of my code.
• [ ] If it is a core feature, I have added thorough tests.
• [ ] Do we need to implement analytics? if so did you add the relevant metrics to the dashboard?
• [ ] If this PR requires public announcement, mark it with /release-notes label and add several sentences in this section.

Checklist before merging

• [ ] Do not forget to reformat commit message to not include the above checklist

[github-actions[bot]]

2922 tests run: 2805 passed, 0 failed, 117 skipped (full report)

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Code coverage* (full report)

• functions: 32.6% (6881 of 21133 functions)
• lines: 50.2% (53658 of 106976 lines)

* collected from Rust tests only

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_{The comment gets automatically updated with the latest test results
fabb29a6339542ee130cd1d32b534fafdc0be240 at 2024-06-25T13:22:36.952Z :recycle:}