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

Without explicitly passing the `media_url` configuration,
Heisenbridge would try to guess it. It works most of the time,
but some people are experiencing trouble with it.

There's no need for wasteful work and for potential unreliability,
so we now configure the `media_url` explicitly.

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2932

Patch contributed by JulianF.

Seems like we don't necessarily need to use a public URL for `SYNCV3_SERVER`.

This possibly helps setups that were having trouble when `SYNCV3_SERVER`
was a public URL (e.g. `https://matrix.DOMAIN`), as described in:
https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2912

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2923

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2919

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/issues/2918

Related to https://github.com/matrix-org/synapse/pull/15488

Fixes https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/2905#issuecomment-1726892215

Many of these do depend on the Synapse master process (`matrix-synapse.service`),
so it makes sense to do it.

Furthermore, we're doing it so that one can stop the `matrix-synapse.service`
service and have systemd cascade this into stopping all the workers as well.

This is useful for easily stopping all of Synapse, so that Postgres
upgrades (`--tags=upgrade-postgres`) can happen cleanly.
Postgres upgrades currently stop `devture_postgres_systemd_services_to_stop_for_maintenance_list` which
includes Synapse, but stopping just the master process and leaving workers running is not safe enough and sometimes leads to errors like:

> ERROR:  insert or update on table "event_forward_extremities" violates foreign key constraint "event_forward_extremities_event_id"

With this dependency in place, stopping `matrix-synapse.service` will stop all Synapse processes.

Related to https://github.com/matrix-org/synapse/pull/16265