fix: fix http admin error, add mimetype for manifest, set corret permission on config.d change

closing this in favor of !28 (merged)

• fix http admin error
• add more comments to nextcloud.conf
• adapt nextcloud.conf to not cache dynamically created css|js|... assets
• add manifest to mimetypes
• add nginx:nginx permissions to conf.d

Small sidenote:

• wrong images sizes specified in core/img/manifest.json (not sure how to adapt them, chrome gives a warning, firefox does not)

E2E Tests:

• the link to the pipline

Manual Performance Tests (when comparing lights.css):

• original setup: it downloaded from sever and loaded into memory each run: 557ms
• setup with changes from this MR: it is loaded once and after that cached in memory for both firefox and chrome. Load time: 0ms