add Dockerfile for simple testing

Dockerfile

@@ -0,0 +1,13 @@
+FROM node:10
+
+# gyp requires `--unsafe-perm`:
+# https://stackoverflow.com/a/52658196/1438024
+RUN npm install -g --unsafe-perm @dollarshaveclub/cloudworker
+
+WORKDIR /app
+COPY . .
+
+EXPOSE 3000
+
+ENTRYPOINT ["cloudworker"]
+CMD ["index.js"]