checker.rb

# checker.rb

# https://blog.codeship.com/build-minimal-docker-container-ruby-apps/

require 'rubygems'
require 'curb'

gem_name = ARGV[0]

raise ArgumentError.new("gem name missing") if gem_name.nil?

if Curl.get("https://rubygems.org/gems/#{gem_name}").status == '200 OK'
  $stdout.puts 'Name not available.'
else
  $stdout.puts 'Name available.'
end

Dockerfile

FROM alpine:3.2
MAINTAINER Leandro López <inkel.ar@gmail.com>

# Install base packages
RUN apk update && apk upgrade

# Install ruby and ruby-bundler
RUN apk add curl-dev ruby-dev build-base ruby

# Clean APK cache
RUN rm -rf /var/cache/apk/*

RUN mkdir /usr/app
WORKDIR /usr/app

COPY . /usr/app

RUN gem install --no-ri --no-rdoc -E curb

ENTRYPOINT ["ruby", "checker.rb"]

gistfile1.txt

$ docker images
REPOSITORY          TAG                 IMAGE ID            CREATED             VIRTUAL SIZE
ruby                latest              7bacacd41c6d        3 minutes ago       183.2 MB

inspect.json

[
{
    "Id": "7bacacd41c6d5cc37b9b0d71796b00cc97673df6cd1e2dc04ab8c98762db5e5a",
    "Parent": "467e02a8493dcadd0061ec26e2f813dbf34b23f77b070ebfc979e01a023b4a5e",
    "Comment": "",
    "Created": "2015-07-20T22:16:41.836628382Z",
    "Container": "c9dea761c45d22c333c67caa6bb98442dffbf11dbea476e6adc12d9a5ff2582d",
    "ContainerConfig": {
        "Hostname": "bfbdc5abbb9f",
        "Domainname": "",
        "User": "",
        "AttachStdin": false,
        "AttachStdout": false,
        "AttachStderr": false,
        "PortSpecs": null,
        "ExposedPorts": null,
        "Tty": false,
        "OpenStdin": false,
        "StdinOnce": false,
        "Env": [
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": [
            "/bin/sh",
            "-c",
            "#(nop) ENTRYPOINT \u0026{[\"ruby\" \"checker.rb\"]}"
        ],
        "Image": "467e02a8493dcadd0061ec26e2f813dbf34b23f77b070ebfc979e01a023b4a5e",
        "Volumes": null,
        "VolumeDriver": "",
        "WorkingDir": "/usr/app",
        "Entrypoint": [
            "ruby",
            "checker.rb"
        ],
        "NetworkDisabled": false,
        "MacAddress": "",
        "OnBuild": [],
        "Labels": {}
    },
    "DockerVersion": "1.7.1",
    "Author": "Leandro López \u003cinkel.ar@gmail.com\u003e",
    "Config": {
        "Hostname": "bfbdc5abbb9f",
        "Domainname": "",
        "User": "",
        "AttachStdin": false,
        "AttachStdout": false,
        "AttachStderr": false,
        "PortSpecs": null,
        "ExposedPorts": null,
        "Tty": false,
        "OpenStdin": false,
        "StdinOnce": false,
        "Env": [
            "PATH=/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin"
        ],
        "Cmd": null,
        "Image": "467e02a8493dcadd0061ec26e2f813dbf34b23f77b070ebfc979e01a023b4a5e",
        "Volumes": null,
        "VolumeDriver": "",
        "WorkingDir": "/usr/app",
        "Entrypoint": [
            "ruby",
            "checker.rb"
        ],
        "NetworkDisabled": false,
        "MacAddress": "",
        "OnBuild": [],
        "Labels": {}
    },
    "Architecture": "amd64",
    "Os": "linux",
    "Size": 0,
    "VirtualSize": 183156199
}
]