test: add test-docker rule (#779)

* test: add test-docker rule

Dockerfile

@@ -1,12 +1,14 @@
-FROM node:8
+FROM node:12.17
 
 RUN mkdir /twilio
 WORKDIR /twilio
 
 ENV NODE_PATH /usr/local/lib/node_modules
 
 COPY lib ./lib
+COPY spec ./spec
+COPY examples ./examples
 COPY index.js .
 COPY package* ./
 
-RUN npm install . -g
+RUN npm install . --include=dev

Makefile

@@ -1,4 +1,4 @@
-.PHONY: test-install install test docs clean
+.PHONY: test-install install test test-docker docs clean
 
 test-install:
 	npm install --only=dev
@@ -11,6 +11,10 @@ install:
 test:
 	npm test
 
+test-docker:
+	docker build -t twilio/twilio-node .
+	docker run twilio/twilio-node npm run ci
+
 docs:
 	npm run jsdoc