Implement two-stage build of the Docker image

Author: Ivan Luzyanin

Dockerfile

@@ -1,3 +1,18 @@
+FROM bigtruedata/sbt:0.13.15-2.12.2 AS builder
+WORKDIR /code
+ADD project/ /code/project
+ADD build.sbt /code
+
+## force sbt to load dependencies and compile interface
+RUN mkdir -p src/main/scala && \
+    echo "object Main {}" > src/main/scala/Main.scala && \
+    sbt compile  && rm -rf src
+
+## copy source code and create an assembly jar
+COPY . /code
+RUN sbt assembly
+
+# Build main image
 FROM openjdk:alpine
 
 LABEL maintainer "Ivan Luzyanin <ivan@acorns.com>"
@@ -9,7 +24,7 @@ RUN addgroup -g 9000 -S code && \
   adduser -S -G code app
 USER app
 
-COPY codeclimate-scalastyle-assembly-0.1.0.jar /usr/src/app/engine-core.jar
+COPY --from=builder /code/target/scala-2.12/codeclimate-scalastyle-assembly-0.1.0.jar /usr/src/app/engine-core.jar
 COPY src/main/resources/docker /usr/src/app
 COPY src/main/resources/docker/engine.json /
 

build.sbt

@@ -1,7 +1,7 @@
 name := "codeclimate-scalastyle"
 organization in ThisBuild := "codeclimate"
 version in ThisBuild := "0.1.0"
-scalaVersion in ThisBuild := "2.12.4"
+scalaVersion in ThisBuild := "2.12.2"
 
 concurrentRestrictions in Global += Tags.limit(Tags.Test, 1)
 parallelExecution in Global := false

codeclimate-scalastyle-assembly-0.1.0.jar

@@ -1 +1 @@
-sbt.version=0.13.16
+sbt.version=0.13.15