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.