Use multistage Dockerfile to cut down image size (#140)

Dockerfile

@@ -1,10 +1,15 @@
+FROM node:8-alpine as builder
+WORKDIR /usr/src/app
+RUN apk add --update build-base python
+COPY . /usr/src/app
+RUN yarn
+
 FROM node:8-alpine
 MAINTAINER butlerx@notthe.cloud
 WORKDIR /app
 RUN adduser -D -h /home/term -s /bin/sh term && \
     ( echo "term:term" | chpasswd ) && \
-    apk add --update build-base python openssh-client
+	apk add openssh-client
 EXPOSE 3000
-COPY . /app
-RUN yarn
+COPY --from=builder /usr/src/app /app
 CMD node bin