feat: add initial multi-environment CI/CD pipeline POC

Adds all required files for the proof of concept:
- Gitea Actions CI/CD workflows for QA, staging, and production environments
- Docker build configuration with healthcheck and runtime environment injection
- Nginx server config with security headers and health endpoint
- Sample static frontend application displaying environment metrics
- Comprehensive README documentation with architecture, setup, and usage instructions

Dockerfile

@@ -0,0 +1,37 @@
+ARG APP_VERSION=0.0.0
+ARG BUILD_DATE=unknown
+ARG GIT_COMMIT=unknown
+ARG GIT_BRANCH=unknown
+
+FROM nginx:alpine AS runtime
+
+ARG APP_VERSION
+ARG BUILD_DATE
+ARG GIT_COMMIT
+ARG GIT_BRANCH
+
+LABEL org.opencontainers.image.title="cicd-multi-env-pipeline-poc"
+LABEL org.opencontainers.image.description="Multi-environment CI/CD pipeline POC"
+LABEL org.opencontainers.image.version="${APP_VERSION}"
+LABEL org.opencontainers.image.created="${BUILD_DATE}"
+LABEL org.opencontainers.image.revision="${GIT_COMMIT}"
+LABEL org.opencontainers.image.source="https://git.kubistudio.cloud/kubistudio/cicd-multi-env-pipeline-poc"
+
+RUN apk add --no-cache curl
+
+COPY nginx.conf /etc/nginx/nginx.conf
+COPY src/ /usr/share/nginx/html/
+COPY healthcheck.sh /usr/local/bin/healthcheck.sh
+
+RUN chmod +x /usr/local/bin/healthcheck.sh
+
+COPY docker-entrypoint.sh /usr/local/bin/
+RUN chmod +x /usr/local/bin/docker-entrypoint.sh
+
+EXPOSE 80
+
+HEALTHCHECK --interval=30s --timeout=5s --start-period=10s --retries=3 \
+  CMD /usr/local/bin/healthcheck.sh
+
+ENTRYPOINT ["/usr/local/bin/docker-entrypoint.sh"]
+CMD ["nginx", "-g", "daemon off;"]