Try to tighten Signal bridge security

roles/matrix-bridge-mautrix-signal/templates/systemd/matrix-mautrix-signal-daemon.service.j2

@@ -21,9 +21,11 @@ ExecStartPre=-{{ matrix_host_command_docker }} rm matrix-mautrix-signal-daemon
 # Intentional delay, so that the homeserver (we likely depend on) can manage to start.
 ExecStartPre={{ matrix_host_command_sleep }} 5
 
+# We can't use `--read-only` for this bridge.
 ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-mautrix-signal-daemon \
 			--log-driver=none \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
 			--network={{ matrix_docker_network }} \
 			-v {{ matrix_mautrix_signal_daemon_path }}:/signald:z \
 			{{ matrix_mautrix_signal_daemon_docker_image }}

roles/matrix-bridge-mautrix-signal/templates/systemd/matrix-mautrix-signal.service.j2

@@ -24,6 +24,8 @@ ExecStart={{ matrix_host_command_docker }} run --rm --name matrix-mautrix-signal
 			--log-driver=none \
 			--network={{ matrix_docker_network }} \
 			--user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
+			--cap-drop=ALL \
+			--read-only \
 			{% if matrix_mautrix_signal_container_http_host_bind_port %}
 			-p {{ matrix_mautrix_signal_container_http_host_bind_port }}:29328 \
 			{% endif %}