add polkadot build script

scripts/build_polkadot.sh

@@ -0,0 +1,22 @@
+#!/usr/bin/env bash
+
+set -e
+
+cumulus_repo=$(cd "$(dirname "$0")" && git rev-parse --show-toplevel)
+polkadot_repo=$(dirname "$cumulus_repo")/polkadot
+if [ ! -d "$polkadot_repo/.git" ]; then
+    echo "please clone polkadot in parallel to this repo:"
+    echo "  (cd .. && git clone [email protected]:paritytech/polkadot.git)"
+    exit 1
+fi
+
+if [ -z "$BRANCH" ]; then
+    BRANCH=bkchr-cumulus-branch
+fi
+
+cd "$polkadot_repo"
+git checkout "$BRANCH"
+time docker build \
+    -f ./docker/Dockerfile \
+    --build-arg PROFILE=release \
+    -t polkadot:"$BRANCH" .