Pass proper UID/GID to Synapse

Fixes a regression caused by a5ee39266c.

If the user id and group id were different than 991:991
(which used to be a hardcoded default for us long ago),
there was a mismatch between what Synapse was trying to use (991:991)
and what it was actually started with (in `--user=..`). It was then
trying to change ownership, which was failing.

This was mostly affecting newer installations which were not using the
991:991 defaults we had long ago (since a1c5a197a9).

roles/matrix-synapse/templates/synapse/systemd/matrix-synapse-worker.service.j2

@@ -17,6 +17,8 @@ ExecStartPre={{ matrix_host_command_sleep }} 5
 ExecStart={{ matrix_host_command_docker }} run --rm --name {{ matrix_synapse_worker_container_name }} \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			-e UID={{ matrix_user_uid }} \
+			-e GID={{ matrix_user_gid }} \
 			--cap-drop=ALL \
 			--read-only \
 			--tmpfs=/tmp:rw,noexec,nosuid,size={{ matrix_synapse_tmp_directory_size_mb }}m \

roles/matrix-synapse/templates/synapse/systemd/matrix-synapse.service.j2

@@ -33,6 +33,8 @@ ExecStartPre={{ matrix_host_command_sleep }} 3
 ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-synapse \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--env=UID={{ matrix_user_uid }} \
+			--env=GID={{ matrix_user_gid }} \
 			--cap-drop=ALL \
 			--read-only \
 			--tmpfs=/tmp:rw,noexec,nosuid,size={{ matrix_synapse_tmp_directory_size_mb }}m \