Make dockerfiles Google Shell Guide Compliant
diff --git a/scripts/ci/dockerfiles/bats/build_and_push.sh b/scripts/ci/dockerfiles/bats/build_and_push.sh
index 61d069480fcb2..5197ca1467577 100755
--- a/scripts/ci/dockerfiles/bats/build_and_push.sh
+++ b/scripts/ci/dockerfiles/bats/build_and_push.sh
@@ -18,18 +18,28 @@
 set -euo pipefail
 DOCKERHUB_USER=${DOCKERHUB_USER:="apache"}
 DOCKERHUB_REPO=${DOCKERHUB_REPO:="airflow"}
+readonly DOCKERHUB_USER
+readonly DOCKERHUB_REPO
+
 BATS_VERSION="1.2.1"
 BATS_ASSERT_VERSION="2.0.0"
 BATS_SUPPORT_VERSION="0.3.0"
 BATS_FILE_VERSION="0.3.0"
+readonly BATS_VERSION
+readonly BATS_ASSERT_VERSION
+readonly BATS_SUPPORT_VERSION
+readonly BATS_FILE_VERSION
 
 AIRFLOW_BATS_VERSION="2020.09.05"
+readonly AIRFLOW_BATS_VERSION
 
 COMMIT_SHA=$(git rev-parse HEAD)
+readonly COMMIT_SHA
 
 cd "$( dirname "${BASH_SOURCE[0]}" )" || exit 1
 
 TAG="${DOCKERHUB_USER}/${DOCKERHUB_REPO}:bats-${AIRFLOW_BATS_VERSION}-${BATS_VERSION}"
+readonly TAG
 
 docker build . \
     --pull \


---
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Read the **[Pull Request Guidelines](https://github.com/apache/airflow/blob/master/CONTRIBUTING.rst#pull-request-guidelines)** for more information.
In case of fundamental code change, Airflow Improvement Proposal ([AIP](https://cwiki.apache.org/confluence/display/AIRFLOW/Airflow+Improvements+Proposals)) is needed.
In case of a new dependency, check compliance with the [ASF 3rd Party License Policy](https://www.apache.org/legal/resolved.html#category-x).
In case of backwards incompatible changes please leave a note in [UPDATING.md](https://github.com/apache/airflow/blob/master/UPDATING.md).
