We do use some `:latest` images by default for the following services:
- matrix-dimension
- Goofys (in the matrix-synapse role)
- matrix-bridge-appservice-irc
- matrix-bridge-appservice-discord
- matrix-bridge-mautrix-facebook
- matrix-bridge-mautrix-whatsapp

It's terribly unfortunate that those software projects don't release
anything other than `:latest`, but that's how it is for now.

Updating that software requires that users manually do `docker pull`
on the server. The playbook didn't force-repull images that it already
had.

With this patch, it starts doing so. Any image tagged `:latest` will be
force re-pulled by the playbook every time it's executed.

It should be noted that even though we ask the `docker_image` module to
force-pull, it only reports "changed" when it actually pulls something
new. This is nice, because it lets people know exactly when something
gets updated, as opposed to giving the indication that it's always
updating the images (even though it isn't).

This doesn't replace all usage of `-v`, but it's a start.

People sometimes troubleshoot by deleting files (especially bridge
config files). Restarting Synapse with a missing registration.yaml file
for a given bridge, causes the `-v
/something/registration.yaml:/something/registration.yaml:ro` option
to force-create `/something/registration.yaml` as a directory.

When a path that's provided to the `-v` option is missing, Docker
auto-creates that path as a directory.
This causes more breakage and confusion later on.

We'd rather fail, instead of magically creating directories.
Using `--mount`, instead of `-v` is the solution to this.

From Docker's documentation:

> When you use --mount with type=bind, the host-path must refer to an existing path on the host.
> The path will not be created for you and the service will fail with an error if the path does not exist.

Fixes #171 (Github Issue).

Fixes #155 (Github Issue)