fix(docker): drop the null environment: block that invalidated compose

`environment:` was present with nothing but comments beneath it, so YAML
parsed it as null and Compose refused the whole file:

    services.mcp.environment must be a mapping

That is every documented Compose command — `docker compose build`,
`docker compose run --rm mcp`, `docker compose down` — failing before
anything starts, on any machine. The README points at this file.

Comment the key out along with its example entries, and say in the file
why it cannot be left bare. The examples switch to mapping syntax so
uncommenting them yields a valid block.

Verified with `docker compose config` on the shipped file (now valid)
and by running the documented flow against a build of this tree: mine a
mounted directory through `docker compose run --rm mcp cli mine`, then
read it back with `... cli search`, which returns the drawer verbatim.
`deploy/docker-compose.server.yml` was checked for the same defect and
is unaffected.

docker-compose.yml

@@ -21,12 +21,15 @@ services:
     tty: false
     volumes:
       - mempalace-data:/data
-    environment:
-      # Everything defaults correctly from HOME=/data (palace ->
-      # /data/.mempalace/palace, config -> /data/.mempalace, model cache ->
-      # /data/.cache). Override here only for non-default locations, e.g.:
-      # - MEMPALACE_EMBEDDING_MODEL=embeddinggemma   # multilingual (default: minilm)
-      # - MEMPALACE_PALACE_PATH=/data/custom/palace
+    # Everything defaults correctly from HOME=/data (palace ->
+    # /data/.mempalace/palace, config -> /data/.mempalace, model cache ->
+    # /data/.cache), so no environment block is needed. Uncomment the key
+    # *together with* an entry under it for non-default locations — a bare
+    # `environment:` with only comments beneath parses as null, and Compose
+    # rejects the whole file with "services.mcp.environment must be a mapping".
+    # environment:
+    #   MEMPALACE_EMBEDDING_MODEL: embeddinggemma   # multilingual (default: minilm)
+    #   MEMPALACE_PALACE_PATH: /data/custom/palace
 
 volumes:
   mempalace-data: