ci(gitlab-ci): Reduce automated triggers

This patch reduces the image builds to a minimum stable releases should
only be build then the `.release` file changes, while deb builds can run
on any commit.

images/dovecot/.gitlab-ci.yaml

@@ -17,7 +17,9 @@ dovecot-container-build:
       .
     - podman push "quay.io/shivering-isles/dovecot:${release}"
   rules:
-    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH && $CI_PIPELINE_SOURCE == "push"'
+      changes:
+        - images/dovecot/.release
 
 dovecot-container-build-dev:
   stage: build
@@ -38,4 +40,6 @@ dovecot-container-build-dev:
       .
     - podman push "quay.io/shivering-isles/dovecot:${CI_COMMIT_SHORT_SHA}"
   rules:
-    - if: '$CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH'
+    - if: '$CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH && $CI_PIPELINE_SOURCE == "push"'
+      changes:
+        - images/dovecot/

images/postfix/.gitlab-ci.yaml

@@ -17,7 +17,9 @@ postfix-container-build:
       .
     - podman push "quay.io/shivering-isles/postfix:${release}"
   rules:
-    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH'
+    - if: '$CI_COMMIT_BRANCH == $CI_DEFAULT_BRANCH && $CI_PIPELINE_SOURCE == "push"'
+      changes:
+        - images/postfix/.release
 
 postfix-container-build-dev:
   stage: build
@@ -38,4 +40,6 @@ postfix-container-build-dev:
       .
     - podman push "quay.io/shivering-isles/postfix:${CI_COMMIT_SHORT_SHA}"
   rules:
-    - if: '$CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH'
+    - if: '$CI_COMMIT_BRANCH != $CI_DEFAULT_BRANCH && $CI_PIPELINE_SOURCE == "push"'
+      changes:
+        - images/postfix/