Multi-worker-docker-container: disable log buffering

Background: we have a `matrixdotorg/synapse-workers` docker image, which is
intended for running multiple workers within the same container. That image
includes a `prefix-log` script which, for each line printed to stdout or stderr
by one of the processes, prepends the name of the process.

This commit disables buffering in that script, so that lines are logged quickly
after they are printed. This makes it much easier to understand the output,
since they then come out in a natural order.

changelog.d/16919.misc

@@ -0,0 +1 @@
+Multi-worker-docker-container: disable log buffering.

docker/prefix-log

@@ -7,6 +7,9 @@
 #   prefix-log command [args...]
 #
 
-exec 1> >(awk '{print "'"${SUPERVISOR_PROCESS_NAME}"' | "$0}' >&1)
-exec 2> >(awk '{print "'"${SUPERVISOR_PROCESS_NAME}"' | "$0}' >&2)
+# '-W interactive' is a `mawk` extension which disables buffering on stdout and sets line-buffered reads on
+# stdin. The effect is that the output is flushed after each line, rather than being batched, which helps reduce
+# confusion due to to interleaving of the different processes.
+exec 1> >(awk -W interactive '{print "'"${SUPERVISOR_PROCESS_NAME}"' | "$0 }' >&1)
+exec 2> >(awk -W interactive '{print "'"${SUPERVISOR_PROCESS_NAME}"' | "$0 }' >&2)
 exec "$@"