Add org.opencontainers labels to Docker images

hooks/pre_build

@@ -2,13 +2,15 @@
 
 set -ex
 
-# Print some environment info in case it's useful for troubleshooting.
-id
-pwd
-df -h
-env
-docker info
-docker version
+# If requested, print some environment info for troubleshooting.
+if [[ -n "${DOCKER_HUB_DEBUG}" ]]; then
+    id
+    pwd
+    df -h
+    env
+    docker info
+    docker version
+fi
 
 # Install build dependencies.
 deps=(
@@ -16,3 +18,11 @@ deps=(
 )
 apt-get update
 apt-get install -y "${deps[@]}"
+
+# Docker Hub uses a shallow clone and doesn't fetch tags, which breaks some
+# Git operations that we perform later, so fetch the complete history and
+# tags first. Note that if the build is cached, the clone may have been
+# unshallowed already; if so, unshallowing will fail, so skip it.
+if [[ -f .git/shallow ]]; then
+    git fetch --unshallow --tags
+fi