Improve docker image size with a 2 stages image (#463)

* Improve docker image size with a 2 stages image

* Minor doc updates

* Fix and reduce size of the docker image

* Fix paths in scripts

substrate/scripts/common.sh

@@ -0,0 +1,25 @@
+#!/usr/bin/env bash
+
+ROOT=`dirname "$0"`
+
+# A list of directories which contain wasm projects.
+SRCS=(
+	"polkadot/runtime/wasm"
+	"substrate/executor/wasm"
+)
+
+DEMOS=(
+	"demo/runtime/wasm"
+	"substrate/test-runtime/wasm"
+	"polkadot/test-parachains/"
+)
+
+# Make pushd/popd silent.
+
+pushd () {
+	command pushd "$@" > /dev/null
+}
+
+popd () {
+	command popd "$@" > /dev/null
+}