Add docker setup to build jar

Author: joachimschmidt557

.dockerignore

@@ -0,0 +1,5 @@
+node_modules
+dist
+dist_keycloak
+# IMPORTANT: Make sure `.gitignore` is **not** listed
+# in your .dockerignore file

Dockerfile

@@ -0,0 +1,10 @@
+FROM node:20-alpine AS build
+RUN apk update && \
+    apk add --no-cache openjdk17 maven
+WORKDIR /app
+COPY . .
+RUN npm ci
+RUN npm run build-keycloak-theme
+
+FROM scratch
+COPY --from=build /app/dist_keycloak/keycloak-theme-for-kc-all-other-versions.jar .