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