Add shared public edge (nginx + certbot)

One nginx owns host ports 80/443 for every project (Gitea, Musify) and
proxies by container name over infra-net and gitea_gitea; a certbot
service renews the Let's Encrypt certificates.

edge/snippets/proxy.conf

@@ -0,0 +1,7 @@
+proxy_http_version 1.1;
+proxy_set_header Host $host;
+proxy_set_header X-Real-IP $remote_addr;
+proxy_set_header X-Forwarded-For $proxy_add_x_forwarded_for;
+proxy_set_header X-Forwarded-Proto $scheme;
+proxy_set_header Upgrade $http_upgrade;
+proxy_set_header Connection $connection_upgrade;