Expose container release as pipeline input

.gitlab-ci.yml

@@ -1,3 +1,12 @@
+spec:
+  inputs:
+    release_version:
+      type: string
+      default: ""
+      regex: '^$|^[0-9]+\.[0-9]+\.[0-9]+$'
+      description: "Server image version to publish (strict SemVer, for example 3.1.0). Leave blank for a test-only pipeline."
+---
+
 stages:
   - test
   - package
@@ -11,9 +20,7 @@ workflow:
     - if: "$CI_COMMIT_BRANCH || $CI_COMMIT_TAG"
 
 variables:
-  RELEASE_VERSION:
+  RELEASE_VERSION: "$[[ inputs.release_version ]]"
-    value: ""
-    description: "Server image version to publish (strict SemVer, for example 3.1.0). Leave blank for a test-only pipeline."
 
 include:
   - local: /.gitlab/ci/tests.yml