Improve Dockerfile FROM

- Fix to actually using argument in ci.Dockerfile
- Unify the format in ci.Dockerfile and Dockerfile
- The new format allows referring to an image by SHA instead of tag. The format to refer to a image by SHA starts with "@sha256:" instead of ":".

Change-Id: If64e0197ae9196bbc52e99098ab29c6ce8284411

Author: mering

Dockerfile

@@ -12,8 +12,8 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
-ARG BAZEL_VERSION=6.0.0
-FROM gcr.io/bazel-public/bazel:${BAZEL_VERSION} AS build
+ARG BAZEL_VERSION=:6.0.0
+FROM gcr.io/bazel-public/bazel${BAZEL_VERSION} AS build
 COPY . .
 RUN bazel build //java/com/google/copybara:copybara_deploy.jar
 

ci.Dockerfile

@@ -12,6 +12,7 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+ARG BAZEL_VERSION
 FROM gcr.io/bazel-public/bazel${BAZEL_VERSION} AS build
 COPY . .
 USER root