add missing gettext dependency in the CI

.gitlab-ci.yml

@@ -20,6 +20,7 @@ build:
           export VERSIONS_TAG="${R_VERSION}--${SHINY_SERVER_VERSION}"
         fi
       fi
+    - apk add gettext
     - envsubst < ./Dockerfile.$LANGUAGE > Dockerfile
     # pull the latest build
     - docker pull "$CI_REGISTRY_IMAGE/${LANGUAGE}:latest" || true