This allows people to augment the Synapse image with custom tools and
addons without having to rebuild it from scratch.
If customizations are enabled, the playbook will build a new
`localhost/matrixdotorg/synapse:VERSION-customized` image
on top of the default one (`FROM matrixdotorg/synapse:VERSION`)
and with custom Dockerfile build steps.
For servers that self-build the Synapse image, the Synapse image will be
built first, before proceding to extend it the same way.
In the future, we'll also have easy to enable Dockerfile build steps
for modules that the playbook supports.
Since we'll likely have generic SQLite database importing
via [pgloader](https://pgloader.io/) for migrating bridge
databases from SQLite to Postgres, we'd rather avoid
calling the "import Synapse SQLite database" command
as just `--tags=import-sqlite-db`.
Similarly, for the media store, we'd like to mention that it's
related to Synapse as well.
We'd like to be more explicit, so as to be less confusing,
especially in light of other homeserver implementations
coming in the future.
Slavi Pantaleev
Marko Weltzer