Add Docker labels.

init/docker/hooks/build

@@ -7,6 +7,10 @@ set -e -o pipefail
 # The Docker Cloud config must pass in the BUILDS env variable.
 # See README.md (in this dir) and the screenshot for more info.
 
+VERSION=$(git tag -l --merged | tail -n1 | tr -d v) # 1.2.3
+DATE=$(date -u +%Y-%m-%dT%H:%M:%SZ)
+COMMIT=$(git rev-parse --short HEAD)
+
 # Build each configured image from Docker Cloud.
 for build in $BUILDS; do
   # os:name:arch:variant
@@ -15,6 +19,9 @@ for build in $BUILDS; do
   echo "Building Image ${IMAGE_NAME}_${os}_${name}"
   docker build \
     --build-arg "ARCH=${name}" \
+    --build-arg "BUILD_DATE=${DATE}" \
+    --build-arg "COMMIT=${COMMIT}" \
+    --build-arg "VERSION=${VERSION}" \
     --tag "${IMAGE_NAME}_${os}_${name}" \
     --file Dockerfile ../..
 done