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/build.sh

@@ -0,0 +1,28 @@
+#!/usr/bin/env bash
+
+# This script assumes that all pre-requisites are installed.
+
+set -e
+
+PROJECT_ROOT=`git rev-parse --show-toplevel`
+source `dirname "$0"`/common.sh
+
+export CARGO_INCREMENTAL=0
+
+# Save current directory.
+pushd .
+
+cd $ROOT
+
+for SRC in "${SRCS[@]}"
+do
+  echo "*** Building wasm binaries in $SRC"
+  cd "$PROJECT_ROOT/$SRC"
+
+  ./build.sh
+
+  cd - >> /dev/null
+done
+
+# Restore initial directory.
+popd