Refine docker image tagging to avoid breaking stabilization branches on updates

ci/docker-rust/Dockerfile

@@ -1,3 +1,5 @@
+# Note: when the rust version (1.28) is changed also modify
+# ci/buildkite.yml to pick up the new image tag
 FROM rust:1.28
 
 RUN set -x && \

ci/docker-rust/build.sh

@@ -3,4 +3,9 @@
 cd "$(dirname "$0")"
 
 docker build -t solanalabs/rust .
+
+read -r rustc version _ < <(docker run solanalabs/rust rustc --version)
+[[ $rustc = rustc ]]
+docker tag solanalabs/rust:latest solanalabs/rust:"$version"
+
 docker push solanalabs/rust