Add ARM Dockerfile (#23)

Dockerfile.armhf

@@ -0,0 +1,25 @@
+FROM resin/raspberrypi3-alpine
+
+RUN [ "cross-build-start" ]
+
+RUN set -x \
+    # Install ngrok (latest official stable from https://ngrok.com/download).
+ && apk add --no-cache curl \
+ && curl -Lo /ngrok.zip https://bin.equinox.io/c/4VmDzA7iaHb/ngrok-stable-linux-arm.zip \
+ && unzip -o /ngrok.zip -d /bin \
+ && rm -f /ngrok.zip \
+    # Create non-root user.
+ && adduser -h /home/ngrok -D -u 6737 ngrok
+
+# Add config script.
+COPY ngrok.yml /home/ngrok/.ngrok2/
+COPY entrypoint.sh /
+
+RUN [ "cross-build-end" ]
+
+USER ngrok
+ENV USER=ngrok
+
+EXPOSE 4040
+
+CMD ["/entrypoint.sh"]