Build test express app

Just so I can test the basic functionality of balena.

Based off of https://github.com/balena-io-projects/simple-server-node/

.gitignore

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+# Log files
+*.log
+
+node_modules

Dockerfile.template

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+FROM balenalib/%%BALENA_MACHINE_NAME%%-node:10-stretch-run
+
+# Defines our working directory in the container
+WORKDIR /usr/src/app
+
+# Copy package.json AND package-lock.json into the container first
+COPY package*.json ./
+
+# Installs npm dependencies on the balena build server, making sure to clean up
+# the artifacts it creates in order to reduce the image size
+RUN JOBS=MAX npm install --production --unsafe-perm && npm cache verify && rm -rf /tmp/*
+
+# Copy all files in our root to the container's working directory
+COPY . ./
+
+# Enable udevd so that plugged dynamic hardware devices can show up in our container
+ENV UDEV=1
+
+# server.js will run when container starts up on the device
+CMD ["npm", "start"]