Dockerfile-workers: reduce the amount we install

This is an attempt to reduce the rebuild time. In short, we reduce the amount
of stuff that the dockerfile installs, so as to give a faster startup.

changelog.d/12464.misc

@@ -0,0 +1 @@
+Dockerfile-workers: reduce the amount we install in the image.

docker/Dockerfile-workers

@@ -2,10 +2,19 @@
 FROM matrixdotorg/synapse
 
 # Install deps
-RUN apt-get update
-RUN apt-get install -y supervisor redis nginx
+RUN \
+   --mount=type=cache,target=/var/cache/apt,sharing=locked \
+   --mount=type=cache,target=/var/lib/apt,sharing=locked \
+  apt-get update && \
+  DEBIAN_FRONTEND=noninteractive apt-get install -y --no-install-recommends \
+     redis-server nginx-light
 
-# Remove the default nginx sites
+# Install supervisord with pip instead of apt, to avoid installing a second
+# copy of python.
+RUN --mount=type=cache,target=/root/.cache/pip \
+    pip install supervisor~=4.2
+
+# Disable the default nginx sites
 RUN rm /etc/nginx/sites-enabled/default
 
 # Copy Synapse worker, nginx and supervisord configuration template files
@@ -19,5 +28,7 @@ EXPOSE 8080/tcp
 COPY ./docker/configure_workers_and_start.py /configure_workers_and_start.py
 ENTRYPOINT ["/configure_workers_and_start.py"]
 
+# Replace the healthcheck with one which checks *all* the workers. The script
+# is generated by configure_workers_and_start.py.
 HEALTHCHECK --start-period=5s --interval=15s --timeout=5s \
     CMD /bin/sh /healthcheck.sh

docker/configure_workers_and_start.py

@@ -512,9 +512,10 @@ def generate_worker_files(environ, config_path: str, data_dir: str):
     )
 
     # Supervisord config
+    os.makedirs("/etc/supervisor", exist_ok=True)
     convert(
         "/conf/supervisord.conf.j2",
-        "/etc/supervisor/conf.d/supervisord.conf",
+        "/etc/supervisor/supervisord.conf",
         main_config_path=config_path,
         worker_config=supervisord_config,
     )
@@ -532,14 +533,6 @@ def generate_worker_files(environ, config_path: str, data_dir: str):
         os.mkdir(log_dir)
 
 
-def start_supervisord():
-    """Starts up supervisord which then starts and monitors all other necessary processes
-
-    Raises: CalledProcessError if calling start.py return a non-zero exit code.
-    """
-    subprocess.run(["/usr/bin/supervisord"], stdin=subprocess.PIPE)
-
-
 def main(args, environ):
     config_dir = environ.get("SYNAPSE_CONFIG_DIR", "/data")
     config_path = environ.get("SYNAPSE_CONFIG_PATH", config_dir + "/homeserver.yaml")
@@ -567,7 +560,13 @@ def main(args, environ):
 
     # Start supervisord, which will start Synapse, all of the configured worker
     # processes, redis, nginx etc. according to the config we created above.
-    start_supervisord()
+    log("Starting supervisord")
+    os.execl(
+        "/usr/local/bin/supervisord",
+        "supervisord",
+        "-c",
+        "/etc/supervisor/supervisord.conf",
+    )
 
 
 if __name__ == "__main__":