Putting each client into its own network was good for isolation,
but it's quite wasteful in terms of the container network pool.

Self-build now requires the new docker buildx/buildkit, so switch
from the ansible docker module to running a docker build command,
like elsewhere.

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

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

`always` is an nginx suffix, which shouldn't have been added to these.

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.

We expect `--tags=start` to handle systemd reloading, so we don't need
to do it manually each time we install/uninstall a .service file.

- forego removing Docker images - it's not effective anyway, because it
  only removes the last version.. which is a drop in the bucket, usually

- do not reload systemd - it's none of our business. `--tags=start`,
  etc., handle this

- combine all uninstall tasks under a single block, which only runs if
  we detect traces (a leftover systemd .service file) of the component.
  If no such .service is detected, we skip them all. This may lead to
  incorect cleanup in rare cases, but is good enough for the most part.

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.