ExecStopPost should allow us to clean up (docker kill + docker rm)
even if the ExecStart (docker run ..) command failed, and not just after
a graceful service stop was initiated.

Source: https://www.freedesktop.org/software/systemd/man/systemd.service.html#ExecStopPost=

Expected to have regressed after https://github.com/spantaleev/matrix-docker-ansible-deploy/pull/1008

This patch comes with its own downsides (as described in the comments
for matrix_prometheus_node_exporter_container_http_host_bind_port),
but at least there's:
- no security issue
- metrics remain readable from matrix-prometheus (even if the network metrics are inaccurate)

A better patch is certainly welcome.

This reverts commit d0cd709c08.

It seems like it doesn't cause any issues for any of these services.

The quotes around "host" for both `--pid` and `--net` were
causing trouble for me:

> docker: --pid: invalid PID mode.

and:

> docker: Error response from daemon: network "host" not found.

I've also changed the `-v` call to `--mount` for consistency with the
rest of the playbook.

By running it in a more privileged container with access to the host network stack and such

Basic system stats, to show stuff the synapse metrics
can't show such as resource usage by bridges, etc

Seems to work fine as well.

This too has only been tested on debian amd64 so far