[BUGFIX] Handle docroot relative paths correctly in ResourceCompressor

Resolves: #82863
Releases: master, 8.7
Change-Id: Iaf775d83c526017ff882d21c8f6d7c5c1f78a038
Reviewed-on: https://review.typo3.org/54481


Tested-by: TYPO3com <no-reply@typo3.com>
Reviewed-by: Susanne Moog <susanne.moog@typo3.org>
Tested-by: Susanne Moog <susanne.moog@typo3.org>
Reviewed-by: Tymoteusz Motylewski <t.motylewski@gmail.com>
Tested-by: Tymoteusz Motylewski <t.motylewski@gmail.com>