Complement: use SQLite by default (#13075)

If no database is configured explicitly, use sqlite.

This means that you don't have to pass any variables into the image.

changelog.d/13075.misc

@@ -0,0 +1 @@
+Merge the Complement testing Docker images into a single, multi-purpose image.

docker/complement/README.md

@@ -7,7 +7,7 @@ so **please don't use this image for a production server**.
 This multi-purpose image is built on top of `Dockerfile-workers` in the parent directory
 and can be switched using environment variables between the following configurations:
 
-- Monolithic Synapse with SQLite (`SYNAPSE_COMPLEMENT_DATABASE=sqlite`)
+- Monolithic Synapse with SQLite (default, or `SYNAPSE_COMPLEMENT_DATABASE=sqlite`)
 - Monolithic Synapse with Postgres (`SYNAPSE_COMPLEMENT_DATABASE=postgres`)
 - Workerised Synapse with Postgres (`SYNAPSE_COMPLEMENT_DATABASE=postgres` and `SYNAPSE_COMPLEMENT_USE_WORKERS=true`)
 

docker/complement/conf/start_for_complement.sh

@@ -31,7 +31,7 @@ case "$SYNAPSE_COMPLEMENT_DATABASE" in
     export START_POSTGRES=true
     ;;
 
-  sqlite)
+  sqlite|"")
     # Configure supervisord not to start Postgres, as we don't need it
     export START_POSTGRES=false
     ;;