paymentservice: Docker image size optimization (#53)

Reduced docker image size to ~153MB (was ~781MB). 
* Uses alpine linux and multi stage builds. 
* Changed `CMD` to `ENTRYPOINT`

src/paymentservice/Dockerfile

@@ -1,7 +1,13 @@
-FROM node:8
-RUN GRPC_HEALTH_PROBE_VERSION=v0.1.0-alpha.1 && \
-    wget -qO/bin/grpc_health_probe https://github.com/grpc-ecosystem/grpc-health-probe/releases/download/${GRPC_HEALTH_PROBE_VERSION}/grpc_health_probe-linux-amd64 && \
-    chmod +x /bin/grpc_health_probe
+FROM node:8-alpine as base
+
+FROM base as builder
+
+# Some packages (e.g. @google-cloud/profiler) require additional
+# deps for post-install scripts
+RUN apk add --update --no-cache \
+    python \
+    make \
+    g++ 
 
 WORKDIR /usr/src/app
 
@@ -9,8 +15,18 @@ COPY package*.json ./
 
 RUN npm install --only=production
 
+FROM base
+
+RUN GRPC_HEALTH_PROBE_VERSION=v0.1.0-alpha.1 && \
+    wget -qO/bin/grpc_health_probe https://github.com/grpc-ecosystem/grpc-health-probe/releases/download/${GRPC_HEALTH_PROBE_VERSION}/grpc_health_probe-linux-amd64 && \
+    chmod +x /bin/grpc_health_probe
+
+WORKDIR /usr/src/app
+
+COPY --from=builder /usr/src/app/node_modules ./node_modules
+
 COPY . .
 
 EXPOSE 50051
 
-CMD [ "node", "index.js" ]
+ENTRYPOINT [ "node", "index.js" ]