Add docker environment for local development (#257)

* Add docker environment

* Add document of docker

* Move docker files

* Change context

* Move git submodule command to entrypoint.sh

* Update README

docker/entrypoint.sh

@@ -0,0 +1,19 @@
+#!/bin/sh
+
+set -eu
+
+git submodule update --init --recursive
+
+# setup
+nimble c -d:release src/setup_nimforum.nim
+./src/setup_nimforum --dev
+
+# build frontend
+nimble c -r src/buildcss
+nimble js -d:release src/frontend/forum.nim
+mkdir -p public/js
+cp src/frontend/forum.js public/js/forum.js
+
+# build backend
+nimble c src/forum.nim
+./src/forum