Use a multi-stage Docker build

Build a slim image by only including the installed and packaged language
server in the final image.

Author: fwcd

Dockerfile

@@ -1,12 +1,16 @@
 # Running this container will start a language server that listens for TCP connections on port 2088
 # Every connection will be run in a forked child process
 
-# Please note that before building the image, you have to build the language server with `./gradlew :server:installDist`
+FROM alpine:3.10 AS builder
+
+WORKDIR /kotlin-language-server
+COPY . .
+RUN ./gradlew :server:installDist
 
 FROM openjdk:10
 
 WORKDIR /
-COPY server/build/install/server .
+COPY --from=builder /kotlin-language-server/server/build/install/server .
 
 EXPOSE 2088