chore: align alloy, diun and couchbase with repo conventions

alloy: move to latest tags (no moving major tag upstream), trim comments to what each line does.
diun: order .env.example like compose.yml.
couchbase: add .env.example, drop the version key, explain the published ports.

couchbase/.env.example

@@ -0,0 +1,5 @@
+# Copy to .env and fill in. Never commit .env.
+#
+#   cp .env.example .env
+
+# No variables: the cluster is configured in the admin console on port 8091.

couchbase/README.md

@@ -5,7 +5,8 @@
 
 ## Networking
 
-Publishes its ports on the host:
+Clients connect to Couchbase directly on its ports, not through a domain on the
+proxy, so it publishes them on the host:
 
 | Ports | Purpose |
 | --- | --- |

couchbase/compose.yml

@@ -1,5 +1,3 @@
-version: '3'
-
 services:
   db:
     image: couchbase