db: make --repositories-file relative to host root

It used to be relative to the --root specified root, but that
causes issues with relative command line filenames and is unintuitive.
Update documentation accordingly. Fixes #10702.
2 jobs for master in 43 seconds
Status Job ID Name Coverage
  Test
passed #222540
x86_64 docker-alpine
test:alpine

00:00:34

passed #222541
x86_64 docker-alpine
test:debian

00:00:41