ci: Create Dockerfile to build a basic container image

Dockerfile

@@ -0,0 +1,31 @@
+FROM node:22 AS fe-build
+
+RUN mkdir /mnt/workspace
+WORKDIR /mnt/workspace
+
+COPY ./frontend /mnt/workspace
+
+RUN npm install --global pnpm \
+    && pnpm install \
+    && pnpm run build \
+
+FROM gradle AS be-build
+
+RUN mkdir /mnt/workspace
+WORKDIR /mnt/workspace
+
+COPY --exclude=./frontend . ./mnt/workspace
+
+RUN ./gradlew -x test build
+
+FROM eclipse-temurin:21-jre-nanoserver
+
+ENV USE_SYSTEM_CA_CERTS=1
+
+RUN mkdir /opt/home-cert-assistant
+WORKDIR /opt/home-cert-assistant
+
+COPY --from=fe-build /mnt/workspace/dist /opt/home-cert-assistant/frontend
+COPY --from=be-build /mnt/workspace/build/home-cert-assistant-0.0.1-SNAPSHOT.jar /opt/home-cert-assistant/
+
+CMD ["java", "-jar", "/opt/home-cert-assistant/.jar"]