Make (most) containers start as non-root

This makes all containers (except mautrix-telegram and
mautrix-whatsapp), start as a non-root user.

We do this, because we don't trust some of the images.
In any case, we'd rather not trust ALL images and avoid giving
`root` access at all. We can't be sure they would drop privileges
or what they might do before they do it.

Because Postfix doesn't support running as non-root,
it had to be replaced by an Exim mail server.

The matrix-nginx-proxy nginx container image is patched up
(by replacing its main configuration) so that it can work as non-root.
It seems like there's no other good image that we can use and that is up-to-date
(https://hub.docker.com/r/nginxinc/nginx-unprivileged is outdated).

Likewise for riot-web (https://hub.docker.com/r/bubuntux/riot-web/),
we patch it up ourselves when starting (replacing the main nginx
configuration).
Ideally, it would be fixed upstream so we can simplify.

roles/matrix-postgres/tasks/import_postgres.yml

@@ -60,12 +60,13 @@
   set_fact:
     matrix_postgres_import_command: >-
       /usr/bin/docker run --rm --name matrix-postgres-import
+      --user={{ matrix_user_uid }}:{{ matrix_user_gid }}
       --network={{ matrix_docker_network }}
       --env-file={{ matrix_postgres_base_path }}/env-postgres-psql
-      -v {{ server_path_postgres_dump }}:{{ server_path_postgres_dump }}:ro
+      -v {{ server_path_postgres_dump }}:/{{ server_path_postgres_dump|basename }}:ro
       --entrypoint=/bin/sh
       {{ matrix_postgres_docker_image_latest }}
-      -c 'cat {{ server_path_postgres_dump }} |
+      -c 'cat /{{ server_path_postgres_dump|basename }} |
       {{ 'gunzip |' if server_path_postgres_dump.endswith('.gz') else '' }}
       psql -v ON_ERROR_STOP=1 -h matrix-postgres'
 

roles/matrix-postgres/tasks/upgrade_postgres.yml

@@ -70,6 +70,7 @@
 - name: Perform Postgres database dump
   command: |
     /usr/bin/docker run --rm --name matrix-postgres-dump \
+    --user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
     --network={{ matrix_docker_network }} \
     --env-file={{ matrix_postgres_base_path }}/env-postgres-psql \
     -v {{ postgres_dump_dir }}:/out \
@@ -104,6 +105,7 @@
 - name: Perform Postgres database import
   command: |
     /usr/bin/docker run --rm --name matrix-postgres-import \
+    --user={{ matrix_user_uid }}:{{ matrix_user_gid }} \
     --network={{ matrix_docker_network }} \
     --env-file={{ matrix_postgres_base_path }}/env-postgres-psql \
     -v {{ postgres_dump_dir }}:/in:ro \