fix(deploy): guard cd and split DOCKER_GID assignment (shellcheck)

cd $COMPOSE_DIR now aborts on failure instead of deploying from the wrong dir;
DOCKER_GID is declared then exported so the subshell exit isn't masked.

deploy/prod-deploy.sh

@@ -39,10 +39,11 @@ STATE_FILE="${STATE_FILE:-/opt/scrabble/DEPLOYED_TAG}"
 PG_USER="${POSTGRES_USER:-scrabble}"
 PG_DB="${POSTGRES_DB:-scrabble}"
 
-cd "$COMPOSE_DIR"
+cd "$COMPOSE_DIR" || { echo "compose dir $COMPOSE_DIR missing"; exit 1; }
 export REGISTRY
 # otelcol joins the host docker group to read the socket; the GID varies per host.
-export DOCKER_GID="$(getent group docker | cut -d: -f3)"
+DOCKER_GID="$(getent group docker | cut -d: -f3)"
+export DOCKER_GID
 
 dc() { docker compose -f docker-compose.yml -f docker-compose.prod.yml "$@"; }
 use_tag() { export TAG="$1"; }