adservice: Reduced docker image size to ~165MB

(down from ~886MB) by switching to alpine and
using multi stage builds

src/adservice/Dockerfile

@@ -1,9 +1,17 @@
-FROM openjdk: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 openjdk:8-alpine as base
+
+FROM base as builder
+
 WORKDIR /app
 
+# This is because the protoc-gen-grpc-java library 
+# is a glibc compiled binary and alpine is
+# based on musl-libc
+RUN  apk add --no-cache --allow-untrusted \
+    -X https://apkproxy.herokuapp.com/sgerrand/alpine-pkg-glibc \
+    glibc \
+    glibc-bin
+
 # Next three steps are for caching dependency downloads
 # to improve subsequent docker build.
 COPY ["build.gradle", "gradlew", "./"]
@@ -12,5 +20,15 @@ RUN ./gradlew downloadRepos
 
 COPY . .
 RUN ./gradlew installDist
+
+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 /app
+COPY --from=builder /app .
+
 EXPOSE 9555
 ENTRYPOINT ["/app/build/install/hipstershop/bin/AdService"]