Dockerfile for tests: align comments with current behavior (#13867)

Signed-off-by: Mathieu Velten <mathieuv@matrix.org>
Co-authored-by: Richard van der Hoff <1389908+richvdh@users.noreply.github.com>

changelog.d/13867.misc

@@ -0,0 +1 @@
+Correct the comments in the complement dockerfile.

docker/complement/Dockerfile

@@ -8,19 +8,15 @@
 
 ARG SYNAPSE_VERSION=latest
 
-# first of all, we create a base image with a postgres server and database,
-# which we can copy into the target image. For repeated rebuilds, this is
-# much faster than apt installing postgres each time.
-#
-# This trick only works because (a) the Synapse image happens to have all the
-# shared libraries that postgres wants, (b) we use a postgres image based on
-# the same debian version as Synapse's docker image (so the versions of the
-# shared libraries match).
-
-# now build the final image, based on the Synapse image.
-
 FROM matrixdotorg/synapse-workers:$SYNAPSE_VERSION
-    # copy the postgres installation over from the image we built above
+    # First of all, we copy postgres server from the official postgres image,
+    # since for repeated rebuilds, this is much faster than apt installing
+    # postgres each time.
+
+    # This trick only works because (a) the Synapse image happens to have all the
+    # shared libraries that postgres wants, (b) we use a postgres image based on
+    # the same debian version as Synapse's docker image (so the versions of the
+    # shared libraries match).
     RUN adduser --system --uid 999 postgres --home /var/lib/postgresql
     COPY --from=postgres:13-bullseye /usr/lib/postgresql /usr/lib/postgresql
     COPY --from=postgres:13-bullseye /usr/share/postgresql /usr/share/postgresql
@@ -28,7 +24,7 @@ FROM matrixdotorg/synapse-workers:$SYNAPSE_VERSION
     ENV PATH="${PATH}:/usr/lib/postgresql/13/bin"
     ENV PGDATA=/var/lib/postgresql/data
 
-    # initialise the database cluster in /var/lib/postgresql
+    # We also initialize the database at build time, rather than runtime, so that it's faster to spin up the image.
     RUN gosu postgres initdb --locale=C --encoding=UTF-8 --auth-host password
 
     # Configure a password and create a database for Synapse