Copy secrets with correct ownership instead of symlinking them

Secrets are owned by root and are not world-readable by default, so
the frontend cannot access them when it’s not running as root.
Not all versions of docker-compose support setting access rights for
secrets, so instead of wrangling with them, just copy secrets and
re-own them.

Signed-off-by: Andrej Shadura <andrew.shadura@collabora.co.uk>