Use POSIX if-clause to trigger docker hub pushing

.gitlab-ci.yml

@@ -147,7 +147,7 @@ build:debug:cargo:x86_64-unknown-linux-musl:
   before_script:
     - docker login -u "$CI_REGISTRY_USER" -p "$CI_REGISTRY_PASSWORD" $CI_REGISTRY
     # Only log in to Dockerhub if the credentials are given:
-    - if [[ -v $DOCKER_HUB ]]; then docker login -u "$DOCKER_HUB_USER" -p "$DOCKER_HUB_PASSWORD" "$DOCKER_HUB"; fi
+    - if [ -n "${DOCKER_HUB}" ]; then docker login -u "$DOCKER_HUB_USER" -p "$DOCKER_HUB_PASSWORD" "$DOCKER_HUB"; fi
   script:
     # Prepare buildx to build multiarch stuff:
     - docker context create 'ci-context'
@@ -160,8 +160,8 @@ build:debug:cargo:x86_64-unknown-linux-musl:
     # Actually create multiarch image:
     - docker buildx build --pull --push --platform "$PLATFORMS" --tag "$GL_IMAGE_TAG" --tag "$GL_IMAGE_TAG-commit-$CI_COMMIT_SHORT_SHA"--file "$DOCKER_FILE" .
     # Only try to push to docker hub, if auth data for dockerhub exists:
-    - if [[ -v $DOCKER_HUB ]];then docker push "$DH_IMAGE_TAG"; fi
-    - if [[ -v $DOCKER_HUB ]];then docker push "$DH_IMAGE_TAG-commit-$CI_COMMIT_SHORT_SHA"; fi
+    - if [ -n "${DOCKER_HUB}" ]; then docker push "$DH_IMAGE_TAG"; fi
+    - if [ -n "${DOCKER_HUB}" ]; then docker push "$DH_IMAGE_TAG-commit-$CI_COMMIT_SHORT_SHA"; fi
 
 build:docker:next:
   extends: .docker-shared-settings