Docker Basics : Running a Hello World Sinatra App in a Container


To learn how to run a simple Sinatra app in a Docker container.


You must already know the basics of creating a Sinatra app and bundler.


Step 1

Install Docker on Mac OS

Step 2

Create a hello directory to work on this tutorial example. Create a Dockerfile in hello directory.

FROM ruby:2.2.1
MAINTAINER Bugs Bunny <>

RUN apt-get update && \
    apt-get install -y net-tools

# Install gems
ENV HOME /root
COPY Gemfile* $APP_HOME/
RUN bundle install

# Upload source

# Start server
CMD ["ruby", "hello.rb"]

Step 3

Create a Gemfile:

source ''

gem 'rack'
gem 'rack-test'
gem 'sinatra'

Step 4

Create hello.rb:

require 'sinatra'

set :bind, ''

get '/' do
  "Hello World #{params[:name]}".strip

Step 5

~ $cd temp/hello/
~/temp/hello $docker build -t bparanj/hello .

If you get this error:

Sending build context to Docker daemon 
Post http:///var/run/docker.sock/v1.19/build?cgroupparent=&cpuperiod=0&cpuquota=0&cpusetcpus=&cpusetmems=&cpushares=0&dockerfile=Dockerfile&memory=0&memswap=0&rm=1&t=bparanj%2Fhello: dial unix /var/run/docker.sock: no such file or directory. Are you trying to connect to a TLS-enabled daemon without TLS?

You must run the init:

~/temp/hello $boot2docker init

You will see:

Latest release for is v1.7.0
Downloading boot2docker ISO image...
Success: downloaded
    to /Users/bparanj/.boot2docker/boot2docker.iso
Generating public/private rsa key pair.
Your identification has been saved in /Users/bparanj/.ssh/id_boot2docker.
Your public key has been saved in /Users/bparanj/.ssh/
The key fingerprint is:
f8:c4:ca:9c:97:9c:a6:fd:74:38:1a:7f:c5:22:35:4d bparanj@Millions.local
The key's randomart image is:
+--[ RSA 2048]----+
|                 |
|              E  |
|             o   |
|       o    o .  |
|      . S  . o   |
|     o * o... o  |
|      = O +..o   |
|       = = o.    |
|      . o.o.     |

Step 6

~/temp/hello $boot2docker up

You will see:

Waiting for VM and Docker daemon to start...
Writing /Users/bparanj/.boot2docker/certs/boot2docker-vm/ca.pem
Writing /Users/bparanj/.boot2docker/certs/boot2docker-vm/cert.pem
Writing /Users/bparanj/.boot2docker/certs/boot2docker-vm/key.pem

To connect the Docker client to the Docker daemon, please set:
    export DOCKER_CERT_PATH=/Users/bparanj/.boot2docker/certs/boot2docker-vm
    export DOCKER_TLS_VERIFY=1
    export DOCKER_HOST=tcp://

Step 7

Set the environment variables.

~/temp/hello $  export DOCKER_CERT_PATH=/Users/bparanj/.boot2docker/certs/boot2docker-vm
~/temp/hello $  export DOCKER_TLS_VERIFY=1
~/temp/hello $  export DOCKER_HOST=tcp://

Step 8

Now build the image.

~/temp/hello $docker build -t bparanj/hello .

You will see:

Sending build context to Docker daemon 92.67 kB
Sending build context to Docker daemon 
Step 0 : FROM ruby:2.2.1
2.2.1: Pulling from ruby
511136ea3c5a: Pull complete 
d338bb63f151: Pull complete 
65688f7c61c4: Pull complete 
b7820d1ee4ee: Pull complete 
938e3817ad84: Pull complete 
2b7102d6f8b7: Pull complete 
13e80363fdbe: Pull complete 
cea0b47616cc: Pull complete 
07765c61a581: Pull complete 
455a7f74b799: Pull complete 
f6d62680d042: Pull complete 
8f4076e0c1e7: Pull complete 
8d3b208c8cb3: Pull complete 
f24a9de1806d: Pull complete 
279be1187a0e: Pull complete 
ef6e4b7dc7cd: Already exists 
ruby:2.2.1: The image you are pulling has been verified. Important: image verification is a tech preview feature and should not be relied on to provide security.
Digest: sha256:5fe48ac5a2b631a72477d1c0e9fa7ab7d73cac56f476319320f88119236d1741
Status: Downloaded newer image for ruby:2.2.1
 ---> ef6e4b7dc7cd
Step 1 : MAINTAINER Tung Nguyen <>
 ---> Running in b77e2655f8d4
 ---> 695684b308e4
Removing intermediate container b77e2655f8d4
Step 2 : RUN apt-get update &&     apt-get install -y net-tools
 ---> Running in 301ed3776103
Get:1 jessie/updates InRelease [63.1 kB]
Get:2 jessie InRelease [134 kB]
Get:3 jessie-updates InRelease [126 kB]
Get:4 jessie/updates/main amd64 Packages [129 kB]
Get:5 jessie/main amd64 Packages [9038 kB]
Get:6 jessie-updates/main amd64 Packages [3616 B]
Fetched 9494 kB in 9s (1006 kB/s)
Reading package lists...
Reading package lists...
Building dependency tree...
Reading state information...
The following NEW packages will be installed:
0 upgraded, 1 newly installed, 0 to remove and 69 not upgraded.
Need to get 225 kB of archives.
After this operation, 803 kB of additional disk space will be used.
Get:1 jessie/main net-tools amd64 1.60-26+b1 [225 kB]
debconf: delaying package configuration, since apt-utils is not installed
Fetched 225 kB in 1s (195 kB/s)
Selecting previously unselected package net-tools.
(Reading database ... 27883 files and directories currently installed.)
Preparing to unpack .../net-tools_1.60-26+b1_amd64.deb ...
Unpacking net-tools (1.60-26+b1) ...
Setting up net-tools (1.60-26+b1) ...
 ---> ef66bfea86c0
Removing intermediate container 301ed3776103
Step 3 : ENV APP_HOME /app
 ---> Running in 4e082ec44bc9
 ---> 8381274803ef
Removing intermediate container 4e082ec44bc9
Step 4 : ENV HOME /root
 ---> Running in 590a5c1130ec
 ---> 7a314bf9dd1f
Removing intermediate container 590a5c1130ec
Step 5 : RUN mkdir $APP_HOME
 ---> Running in 38153ae9ade1
 ---> 151f938534b3
Removing intermediate container 38153ae9ade1
 ---> Running in 826a540a37fa
 ---> 9bc90cc0be66
Removing intermediate container 826a540a37fa
Step 7 : COPY Gemfile* $APP_HOME/
 ---> 2b1d0e4460f1
Removing intermediate container 13eedae4464d
Step 8 : RUN bundle install
 ---> Running in 7626c821f248
Don't run Bundler as root. Bundler can ask for sudo if it is needed, and
installing your bundle as root will break this application for all non-root
users on this machine.
Fetching gem metadata from
Fetching version metadata from
Installing rack 1.4.1
Installing rack-protection 1.5.3
Installing rack-test 0.6.3
Installing tilt 1.4.1
Installing sinatra 1.4.5
Using bundler 1.9.2
Bundle complete! 3 Gemfile dependencies, 6 gems now installed.
Bundled gems are installed into /usr/local/bundle.
 ---> c1a40addc15e
Removing intermediate container 7626c821f248
Step 9 : COPY . $APP_HOME
 ---> 4d17d8ca2e5a
Removing intermediate container b842914afe96
Step 10 : ENV PORT 3000
 ---> Running in ea7a0b4322b7
 ---> c9a1091260cc
Removing intermediate container ea7a0b4322b7
Step 11 : EXPOSE 3000
 ---> Running in 00e4a9631d92
 ---> 6f786344cb53
Removing intermediate container 00e4a9631d92
Step 12 : CMD ruby hello.rb
 ---> Running in e255db640718
 ---> d881ae171075
Removing intermediate container e255db640718
Successfully built d881ae171075

Step 9

Run the Sinatra app in the container.

~/temp/hello $docker run --rm -p 3000:3000 bparanj/hello

You will see:

[2015-06-20 10:53:23] INFO  WEBrick 1.3.1
[2015-06-20 10:53:23] INFO  ruby 2.2.1 (2015-02-26) [x86_64-linux]
== Sinatra/1.4.5 has taken the stage on 3000 for development with backup from WEBrick
[2015-06-20 10:53:23] INFO  WEBrick::HTTPServer#start: pid=1 port=3000 - - [20/Jun/2015 10:54:06] "GET / HTTP/1.1" 200 11 0.0067 - - [20/Jun/2015:10:54:06 UTC] "GET / HTTP/1.1" 200 11
- -> / - - [20/Jun/2015 10:54:07] "GET /favicon.ico HTTP/1.1" 404 453 0.0007 - - [20/Jun/2015:10:54:07 UTC] "GET /favicon.ico HTTP/1.1" 404 453 -> /favicon.ico

Step 10

For Mac OS:

$boot2docker ip

Use the ip from the above command to view the application on a browser:


In this article, we created a Dockerfile and built an image based on this Dockerfile. Then we ran a simple Sinatra app in a Docker container.

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