Automatically force-pull :latest images

We do use some `:latest` images by default for the following services:
- matrix-dimension
- Goofys (in the matrix-synapse role)
- matrix-bridge-appservice-irc
- matrix-bridge-appservice-discord
- matrix-bridge-mautrix-facebook
- matrix-bridge-mautrix-whatsapp

It's terribly unfortunate that those software projects don't release
anything other than `:latest`, but that's how it is for now.

Updating that software requires that users manually do `docker pull`
on the server. The playbook didn't force-repull images that it already
had.

With this patch, it starts doing so. Any image tagged `:latest` will be
force re-pulled by the playbook every time it's executed.

It should be noted that even though we ask the `docker_image` module to
force-pull, it only reports "changed" when it actually pulls something
new. This is nice, because it lets people know exactly when something
gets updated, as opposed to giving the indication that it's always
updating the images (even though it isn't).

roles/matrix-nginx-proxy/defaults/main.yml

@@ -2,8 +2,9 @@ matrix_nginx_proxy_enabled: true
 
 # We use an official nginx image, which we fix-up to run unprivileged.
 # An alternative would be an `nginxinc/nginx-unprivileged` image, but
-# those as more frequently out of date.
+# that is frequently out of date.
 matrix_nginx_proxy_docker_image: "nginx:1.15.12-alpine"
+matrix_nginx_proxy_docker_image_force_pull: "{{ matrix_nginx_proxy_docker_image.endswith(':latest') }}"
 
 matrix_nginx_proxy_base_path: "{{ matrix_base_data_path }}/nginx-proxy"
 matrix_nginx_proxy_data_path: "{{ matrix_nginx_proxy_base_path }}/data"
@@ -144,6 +145,7 @@ matrix_ssl_domains_to_obtain_certificates_for: []
 # Controls whether to obtain production or staging certificates from Let's Encrypt.
 matrix_ssl_lets_encrypt_staging: false
 matrix_ssl_lets_encrypt_certbot_docker_image: "certbot/certbot:v0.33.1"
+matrix_ssl_lets_encrypt_certbot_docker_image_force_pull: "{{ matrix_ssl_lets_encrypt_certbot_docker_image.endswith(':latest') }}"
 matrix_ssl_lets_encrypt_certbot_standalone_http_port: 2402
 matrix_ssl_lets_encrypt_support_email: ~