Pin Synapse version until read-after-write issue is fixed

Relevant Synapse issue: matrix-org/synapse#15526

Full context: #208 (comment)

test/dockerfiles/Synapse.Dockerfile

@@ -4,7 +4,11 @@
 # Currently this is based on Complement Synapse images which are based on the
 # published 'synapse:latest' image -- ie, the most recent Synapse release.
 
-ARG SYNAPSE_VERSION=latest
+# FIXME: We're pinning the version to `v1.79.0` until
+# https://github.com/matrix-org/synapse/issues/15526 is fixed. Feel free to update back
+# to `latest` once that issue is resolved. More context:
+# https://github.com/matrix-org/matrix-public-archive/pull/208#discussion_r1183294630
+ARG SYNAPSE_VERSION=v1.79.0
 
 FROM matrixdotorg/synapse:${SYNAPSE_VERSION}