feat: add BUILD_VAR_REF_LABEL placeholder

pr-<n> for pull requests, otherwise GITHUB_REF_NAME with / -> -.
Matches the docker-build image tag; existing placeholders unchanged.

Co-Authored-By: Claude Opus 5.5 <[email protected]>

action.yml

@@ -13,6 +13,16 @@ runs:
         SHA_SHORT="${GITHUB_SHA:0:7}"
         BUILD_DATE="$(date -u '+%Y-%m-%dT%H:%M:%SZ')"
 
+        # Same label as the docker-build image tag: pr-<n> for PRs, else ref name with / -> -
+        REF="${GITHUB_REF:-}"
+        if [[ "$REF" == refs/pull/* ]]; then
+          PR_NUM="${REF#refs/pull/}"
+          REF_LABEL="pr-${PR_NUM%%/*}"
+        else
+          REF_NAME="${GITHUB_REF_NAME:-}"
+          REF_LABEL="${REF_NAME//\//-}"
+        fi
+
         declare -A VARS=(
           [BUILD_VAR_GITHUB_SHA]="${GITHUB_SHA:-}"
           [BUILD_VAR_GITHUB_REF_NAME]="${GITHUB_REF_NAME:-}"
@@ -32,6 +42,7 @@ runs:
           [BUILD_VAR_GITEA_ACTIONS_RUNNER_VERSION]="${GITEA_ACTIONS_RUNNER_VERSION:-}"
           [BUILD_VAR_COMMIT_SHA_SHORT]="$SHA_SHORT"
           [BUILD_VAR_BUILD_DATE]="$BUILD_DATE"
+          [BUILD_VAR_REF_LABEL]="$REF_LABEL"
         )
 
         SED_ARGS=()