CI: Configure Travis to build example rootfs

.travis.yml

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+dist: trusty
+sudo: required
+language: minimal
+script:
+  - sudo ./alpine-make-rootfs
+        --branch v3.7
+        --packages 'ruby ruby-bigdecimal sqlite'
+        --script-chroot
+        example-$(date +%Y%m%d).tar.xz -- ./example/install.sh
+#deploy:
+#  provider: releases
+#  api_key:
+#    secure: <Encrypted GitHub Personal Token>
+#  file: "*.tar.xz"
+#  file_glob: true
+#  skip_cleanup: true
+#  on:
+#    tags: true

example/hello_world.rb

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+#!/usr/bin/env ruby
+
+puts 'Hello, world!'

example/install.sh

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+#!/bin/sh
+
+# Copy some file from the repository root to the rootfs.
+install -D -m 755 examples/hello_world.rb /app/hello_world.rb
+
+# Install some dev packages and gem mailcatcher.
+apk add --no-progress -t .make build-base ruby-dev sqlite-dev
+gem install --no-document mailcatcher
+
+# Clean-up dev packages.
+apk del --no-progress .make