add Dockerfile, update README

Dockerfile

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+FROM node:12-alpine
+
+ARG REDIS_URL
+ARG DEBUG
+ARG EPHEMERAL_STORAGE
+
+RUN apk --no-cache --virtual .build-deps add git python build-base
+RUN apk --no-cache --virtual .canvas-deps add cairo-dev pango-dev jpeg-dev giflib-dev
+
+RUN mkdir -p /crafatar/images/faces
+RUN mkdir -p /crafatar/images/helms
+RUN mkdir -p /crafatar/images/skins
+RUN mkdir -p /crafatar/images/renders
+RUN mkdir -p /crafatar/images/capes
+
+VOLUME /crafatar/images
+
+COPY package.json www.js crafatar/
+COPY config.example.js crafatar/config.js
+COPY lib/ crafatar/lib/
+
+WORKDIR /crafatar
+
+RUN npm install
+
+EXPOSE 3000
+ENTRYPOINT npm start