Use a template for generating Dockerfiles

Since the firefox and chrome builds are similar except for the TLS
library used, it makes sense to unify their Dockerfiles. This commit
introduces a template Dockerfile from which both the build Dockerfiles
are generated using the simple 'mustache' template system.

generate_dockerfiles.sh

@@ -0,0 +1,13 @@
+#!/bin/sh
+
+cat <<EOF | mustache - Dockerfile.template > chrome/Dockerfile
+---
+chrome: true
+---
+EOF
+
+cat <<EOF | mustache - Dockerfile.template > firefox/Dockerfile
+---
+firefox: true
+---
+EOF