fix(share): absolute host bind for serve config

Portainer's git-stack checkout does not materialize sibling repo files
at the relative bind path, so ./serve-roms.json mounted as an empty
auto-created directory and tailscale serve failed ('is a directory').
Switch to an absolute /config/share/serve-roms.json bind (matching every
other stack's /config convention); the file is mirrored on the host.

stacks/share/docker-compose.yml

@@ -15,8 +15,10 @@
 #            "tagOwners": { "tag:share": ["autogroup:admin"] }.
 #
 # To share a NEW service:
-#   1. add a `serve-<svc>.json` (Proxy -> http://<container>:<port>)
-#   2. add a `ts-<svc>` service block below (copy ts-roms), join the svc network
+#   1. add a `serve-<svc>.json` here (Proxy -> http://<container>:<port>) AND
+#      mirror it to /config/share/serve-<svc>.json on valhalla (see bind note below)
+#   2. add a `ts-<svc>` service block below (copy ts-roms), join the svc network,
+#      bind /config/share/serve-<svc>.json -> /config/serve.json
 #   3. add that network under `networks:` as external
 #   4. push, then admin console -> Machines -> <svc>-share -> Share -> friend email
 
@@ -37,7 +39,12 @@ services:
       - TS_EXTRA_ARGS=--advertise-tags=tag:share
     volumes:
       - /config/share/ts-roms:/var/lib/tailscale
-      - ./serve-roms.json:/config/serve.json:ro
+      # Absolute host bind: Portainer's git-stack checkout does NOT materialize
+      # sibling repo files at the relative bind path (Docker then auto-creates a
+      # DIRECTORY and serve fails with "is a directory"). The serve config is
+      # canonical in this repo (serve-roms.json) and mirrored to the host path
+      # below — update both when it changes (same model as Caddyfile).
+      - /config/share/serve-roms.json:/config/serve.json:ro
 
 networks:
   roms: