feat: add Dockerfile and Gitea Actions CI pipeline

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,16 @@
+# Dockerfile
+FROM eclipse-temurin:21-jdk-alpine AS build
+WORKDIR /build
+COPY .mvn/ .mvn/
+COPY mvnw pom.xml ./
+RUN ./mvnw dependency:go-offline -B
+COPY src/ src/
+RUN ./mvnw package -DskipTests -B
+
+FROM eclipse-temurin:21-jre-alpine
+WORKDIR /app
+RUN addgroup -S cameleer && adduser -S cameleer -G cameleer
+COPY --from=build /build/target/*.jar app.jar
+USER cameleer
+EXPOSE 8080
+ENTRYPOINT ["java", "-jar", "app.jar"]