add dockerfile

from https://github.com/DiffuseHyperion/Ultraviolet-App-docker

Dockerfile

@@ -0,0 +1,19 @@
+FROM node:18-alpine
+
+ENV NODE_ENV=production
+ARG NPM_BUILD="npm install --omit=dev"
+EXPOSE 8080/tcp
+
+LABEL maintainer="TitaniumNetwork Ultraviolet Team"
+LABEL summary="Ultraviolet Proxy Image"
+LABEL description="Example application of Ultraviolet which can be deployed in production."
+
+WORKDIR /app
+
+COPY ["package.json", "package-lock.json", "./"]
+RUN $NPM_BUILD
+
+COPY . .
+
+ENTRYPOINT [ "node" ]
+CMD ["src/index.js"]