ci(postfix): Fix wrong directory, causing wrong image

This patch updates the gitlab-ci config to actually build a postfix
image for non-main branches. Previous copy-paste mistake should be fixed
by this.

images/postfix/.gitlab-ci.yaml

@@ -26,7 +26,7 @@ postfix-container-build-dev:
     - podman login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
   script:
     - |
-      cd images/dovecot
+      cd images/postfix
       podman build --pull \
       --label "org.opencontainers.image.source=$CI_PROJECT_URL/-/tree/$CI_COMMIT_SHA/images/postfix" \
       --label "org.opencontainers.image.revision=$CI_COMMIT_SHA" \