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-demos.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 $ROOT
+
+for DEMO in "${DEMOS[@]}"
+do
+  echo "*** Building wasm binaries in $DEMO"
+  cd "$PROJECT_ROOT/$DEMO"
+
+  ./build.sh
+
+  cd - >> /dev/null
+done
+
+# Restore initial directory.
+popd

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

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
+}

substrate/scripts/init.sh

@@ -0,0 +1,13 @@
+#!/usr/bin/env bash
+
+set -e
+
+echo "*** Initialising WASM build environment"
+
+rustup update nightly
+rustup target add wasm32-unknown-unknown --toolchain nightly
+rustup update stable
+
+# Install wasm-gc. It's useful for stripping slimming down wasm binaries.
+command -v wasm-gc || \
+	cargo +nightly install --git https://github.com/alexcrichton/wasm-gc