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/Dockerfile

@@ -0,0 +1,14 @@
+FROM nimlang/nim:1.2.6-ubuntu
+
+RUN apt-get update -yqq \
+    && apt-get install -y --no-install-recommends \
+               libsass-dev \
+               sqlite3 \
+    && apt-get clean \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /app
+COPY . /app
+
+# install dependencies
+RUN nimble install -Y