Build the image instead of pulling it on deploy

Arcane pulls every service's image: reference before deploying, and
playlink isn't published to a registry. Drop the image name and set
pull_policy: build so the image is always built from the repository.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.env.example

@@ -3,8 +3,7 @@
 # Anything left out uses the default shown here.
 
 # ---- container ----
-# Image tag to build/run and the container's name.
-PLAYLINK_IMAGE=playlink:latest
+# The container's name. (The image is built from this repository.)
 PLAYLINK_CONTAINER_NAME=playlink
 # Version label shown in the UI footer.
 PLAYLINK_VERSION=dev