Hardening of the Frontend docker image (#377)

* move the env-config script to a sub folder
* fix doc
* fix ports and ref to the official image
* add hardening to the docker-compose examples

frontend/scripts/env.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+
+# This script is used when the docker container starts and does the magic to
+# bring the ENV variables to the generated static UI.
+
+ENV_DIR=./tmp
+mkdir -p "$ENV_DIR"
+TARGET="$ENV_DIR/env-config.js"
+
+# Recreate config file
+echo -n > $TARGET
+
+declare -a vars=(
+  "SUBSTRATE_TELEMETRY_URL"
+  "SUBSTRATE_TELEMETRY_SAMPLE"
+)
+
+echo "window.process_env = {" >> $TARGET
+for VAR in ${vars[@]}; do
+  echo "  $VAR: \"${!VAR}\"," >> $TARGET
+done
+echo "}" >> $TARGET
+
+chmod 440 $TARGET