build: moved jlink build inside the Docker image build, so building t…

…he Docker image works on any host OS

Problem was that jlink produces OS-specific artifacts, so you couldn't make a functional Docker image on Windows.
Final Docker image size has also been reduced, roughly from 240MB to 170MB, by using Alpine as the new base image.
Support for Debian Stretch ended on the 30th of June 2022.

.dockerignore

@@ -0,0 +1,6 @@
+.git
+.gradle
+build
+log
+mongo_data
+pswgcommon/build

.github/workflows/deploy.yml

@@ -13,12 +13,6 @@ jobs:
     - uses: actions/checkout@v1
       with:
         submodules: true
-    - name: Set up JDK 18
-      uses: actions/setup-java@v1
-      with:
-        java-version: 18
-    - name: Build
-      run: ./gradlew --no-daemon clean test jlink
     - name: Docker Login
       run: docker login docker.pkg.github.com -u "${{ secrets.DOCKER_USER }}" -p "${{ secrets.DOCKER_PASS }}"
     - name: Docker Build & Push

Dockerfile

@@ -1,15 +1,21 @@
-FROM debian:stretch-slim
+FROM eclipse-temurin:18.0.2.1_1-jdk-alpine AS builder
+WORKDIR /holocore
+ADD . /holocore
+
+RUN ./gradlew --no-daemon jlink
+
+FROM alpine:3.18.3 AS runner
 
 # Adds necessary Holocore files
 RUN mkdir /holocore
-ADD build/holocore/ /holocore
+COPY --from=builder /holocore/build/holocore/ /holocore
 ADD serverdata/ /holocore/serverdata
 
 # Sets up networking
 EXPOSE 44463/tcp
 
 # Sets up timezone - default timezone can be overridden by setting TZ environment variable.
-RUN apt-get install -y tzdata
+RUN apk add --no-cache tzdata
 ENV TZ=UTC
 
 # Sets up execution