Dockerfile: remove extraneous permission modification commands

[sudoforge]

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Dockerfile: remove extraneous permission modification commands

These are no longer necessary due to the fact that the second `RUN`
instruction runs as the `linuxbrew` user (as of commit
77afe94446e78e688ebde7a5e2b1249fc0a4afcd).

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

[sudoforge]

@dawidd6 please review/merge this before #11819.

[dawidd6]

I don't think you've rebased correctly.

[sudoforge]

Whoops, you're correct -- I rebased on top of my fork's head branch, which caused those commits to be picked up (because the hashes differ). This has now been fixed.

[MikeMcQuaid]

Thanks again @sudoforge!