Dockerfile: use relative paths when in the /home/linuxbrew directory

[sudoforge]

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• Have you checked to ensure there aren't other open Pull Requests for the same change?
• Have you added an explanation of what your changes do and why you'd like us to include them?
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Dockerfile: use relative paths when in the /home/linuxbrew directory

Because the `WORKDIR` instruction specifies the home directory of the
`linuxbrew` user, there is no need to use absolute paths to reference
paths within the home directory, nor is there a need to call `cd` (this
is actually an anti-pattern [0]) to move into a subdirectory before
creating folders.

[0]: https://github.com/hadolint/hadolint/wiki/DL3003

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THIS PR DEPENDS ON #11815 AND #11818 . DO NOT MERGE UNTIL IT IS MARKED READY

[dawidd6]

Could you rebase on latest master?

[MikeMcQuaid]

Thanks again @sudoforge!

[sudoforge]

Happy to help make things a touch easier :)