This is needed when using a prefix path that doesn't end with a slash.

Signed-off-by: Sergio Durigan Junior <sergiodj@sergiodj.net>

Previously, it had to go through matrix-nginx-proxy.
It's exposed to Traefik directly via container labels now

Serving at a path other than `/` doesn't work well yet.

We were mounting our own configuration to
`/usr/share/nginx/html/config.json`, which is a symlink to
`/tmp/config.json`. So we effectively mount our file to
`/tmp/config.json`.

When starting:

- if Hydrogen sees a `CONFIG_OVERRIDE` environment variable,
  it will try to save it into our read-only config file and fail.

- if Hydrogen doesn't see a `CONFIG_OVERRIDE` environment variable (the
  path we go through, because we don't pass such a variable),
  it will try to copy its bundled configuration (`/config.json.bundled`)
  to `/tmp/config.json`. Because our configuration is mounted as read-only, it will
  fail.

In both cases, it will fail with:

> cp: can't create '/tmp/config.json': File exists

Source: 3720de36bb/docker/dynamic-config.sh

We work around this by mounting our configuration on top of the bundled
one (`/config.json.bundled`). We then let Hydrogen's startup script copy
it to `/tmp/config.json` (a tmpfs we've mounted into the container) and use it from there.

This paves the way for installing other roles into `roles/galaxy` using `ansible-galaxy`,
similar to how it's done in:

- https://github.com/spantaleev/gitea-docker-ansible-deploy
- https://github.com/spantaleev/nextcloud-docker-ansible-deploy

In the near future, we'll be removing a lot of the shared role code from here
and using upstream roles for it. Some of the core `matrix-*` roles have
already been extracted out into other reusable roles:

- https://github.com/devture/com.devture.ansible.role.postgres
- https://github.com/devture/com.devture.ansible.role.systemd_docker_base
- https://github.com/devture/com.devture.ansible.role.timesync
- https://github.com/devture/com.devture.ansible.role.vars_preserver
- https://github.com/devture/com.devture.ansible.role.playbook_runtime_messages
- https://github.com/devture/com.devture.ansible.role.playbook_help

We just need to migrate to those.