Docker: support SYNAPSE_WORKER envvar (#6058)

* Allow passing SYNAPSE_WORKER envvar

* changelog.d

* Document SYNAPSE_WORKER.

Attempting to imply that you don't need to change this default
unless you're in worker mode.

Also aware that there's a bigger problem of attempting to document
a complete working configuration of workers using docker, as we
currently only document to use `synctl` for worker mode, and synctl
doesn't work that way in docker.

changelog.d/6058.docker

+Provide SYNAPSE_WORKER envvar to specify python module.

docker/README.md

@@ -89,6 +89,8 @@ The following environment variables are supported in run mode:
   `/data`.
 * `SYNAPSE_CONFIG_PATH`: path to the config file. Defaults to
   `<SYNAPSE_CONFIG_DIR>/homeserver.yaml`.
+* `SYNAPSE_WORKER`: module to execute, used when running synapse with workers.
+   Defaults to `synapse.app.homeserver`, which is suitable for non-worker mode.
 * `UID`, `GID`: the user and group id to run Synapse as. Defaults to `991`, `991`.
 * `TZ`: the [timezone](https://en.wikipedia.org/wiki/List_of_tz_database_time_zones) the container will run with. Defaults to `UTC`.
 

docker/start.py

@@ -182,6 +182,7 @@ def main(args, environ):
     mode = args[1] if len(args) > 1 else None
     desired_uid = int(environ.get("UID", "991"))
     desired_gid = int(environ.get("GID", "991"))
+    synapse_worker = environ.get("SYNAPSE_WORKER", "synapse.app.homeserver")
     if (desired_uid == os.getuid()) and (desired_gid == os.getgid()):
         ownership = None
     else:
@@ -245,7 +246,7 @@ def main(args, environ):
 
     log("Starting synapse with config file " + config_path)
 
-    args = ["python", "-m", "synapse.app.homeserver", "--config-path", config_path]
+    args = ["python", "-m", synapse_worker, "--config-path", config_path]
     if ownership is not None:
         args = ["su-exec", ownership] + args
         os.execv("/sbin/su-exec", args)