Don't require Gradle to build Docker image

Update Dockerfile to use a multi-stage build and run Gradle *within* the
builder container, rather than on the host, so the host needn't have
Gradle or JDK installed, then copy the build .jar from the builder
container into the final container.

Update build.gradle's dockerBuild target to not build the jar, but
instead do it within the Docker build process. This results in some
double-handling because it requires Gradle and JDK both on the host
system *and* within the container, and runs two instances of Gradle
during the build, but is added for backwards compatibility. The better
approach (rather than running `./gradlew dockerBuild`) is to manually
run `docker build -t ma1ua/ma1sd .`.

I've tested this process on both a Debian amd64 system and a Raspberry
Pi 3 running Raspbian, and it seems to work (though the RPi is pretty
slow).

Dockerfile

@@ -1,3 +1,11 @@
+FROM openjdk:8-jre-alpine AS builder
+
+RUN apk update && apk add gradle git && rm -rf /var/lib/apk/* /var/cache/apk/*
+
+WORKDIR /ma1sd
+COPY . .
+RUN ./gradlew shadowJar
+
 FROM openjdk:8-jre-alpine
 
 RUN apk update && apk add bash && rm -rf /var/lib/apk/* /var/cache/apk/*
@@ -15,4 +23,4 @@ CMD [ "/start.sh" ]
 
 ADD src/docker/start.sh /start.sh
 ADD src/script/ma1sd /app/ma1sd
-ADD build/libs/ma1sd.jar /app/ma1sd.jar
+COPY --from=builder /ma1sd/build/libs/ma1sd.jar /app/ma1sd.jar

build.gradle

@@ -264,7 +264,7 @@ task debBuild(dependsOn: shadowJar) {
     }
 }
 
-task dockerBuild(type: Exec, dependsOn: shadowJar) {
+task dockerBuild(type: Exec) {
     commandLine 'docker', 'build', '-t', dockerImageTag, project.rootDir
 
     doLast {