Creating a Dockerfile

Introduction

You can launch a container from a docker image, modify the container, and commit the changes back to the image.  The advantage to this approach is it's likely easier for a beginner - once inside docker you have access to the terminal session and can install and configure software in a manner nearly resembling a full Linux environment.

The alternative is to modify your Dockerfile to contain the commands for installing and configuring file.  This appproach is preferred, because the Dockerfile can be easily checked into a SCM and built on other machines.

Creating a Dockerfile (Basics)

I'm going to create a directory in my home folder called "mydocker" and start editing a file within that directory called "Dockerfile".

My first Dockerfile is simple:

FROM ubuntu

RUN touch myfile.txt

and I can build it using this command:

sudo docker build -t basehadoop .

"basehadoop" is the name I give my image.
"." tells docker to look inside my current directory ("mydocker") to find the Dockerfile.

I can launch a container from the image that I've built using this command:

sudo docker run -i -t basehadoop /bin/bash

and once inside I notice the file I've created exists:

root@74e10ef8405d:/# ls
bin  boot  dev  etc  home  lib  lib64  media  mnt  myfile.txt  opt  proc  root  run  sbin  srv  sys  tmp  usr  var

Setting Environment Variables

Use this pattern in your Dockerfile:

ENV key value

An environment variable like this

export JAVA_HOME=/usr/lib/jvm/java-8-oracle/jre

becomes

ENV JAVA_HOME /usr/lib/jvm/java-8-oracle/jre

Likewise, the PATH can be modified, using this format:

ENV PATH /usr/lib/jvm/java-8-oracle/jre/bin:$PATH

When I re-build my image with this change, I get this:

craig@dockerhost:~/mydocker$ sudo docker build -t basehadoop .
Sending build context to Docker daemon 3.072 kB
Sending build context to Docker daemon 
Step 0 : FROM ubuntu
 ---> b39b81afc8ca
Step 1 : RUN touch myfile.txt
 ---> Using cache
 ---> 0e83d857ff15
Step 2 : ENV JAVA_HOME /usr/lib/jvm/java-8-oracle/jre
 ---> Running in 1204292998bf
 ---> 90808840643c
Removing intermediate container 8cf05deb31d5
Successfully built 849723f3f1d2

When I view the json file under the graphs folder, I see this JSON:

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83

{ "Size": 0, "architecture": "amd64", "config": { "AttachStderr": false, "AttachStdin": false, "AttachStdout": false, "Cmd": [ "/bin/bash" ], "CpuShares": 0, "Cpuset": "", "Domainname": "", "Entrypoint": null, "Env": [ "PATH=/usr/lib/jvm/java-8-oracle/jre/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", "JAVA_HOME=/usr/lib/jvm/java-8-oracle/jre" ], "ExposedPorts": null, "Hostname": "2338b11efbe1", "Image": "90dd9b6eab1d605a10e151660f2a841c4bc64978fb85d3424f97bdb40c94be28", "Memory": 0, "MemorySwap": 0, "NetworkDisabled": false, "OnBuild": [], "OpenStdin": false, "PortSpecs": null, "StdinOnce": false, "Tty": false, "User": "", "Volumes": null, "WorkingDir": "" }, "container": "5c5de1bd02358774ee5766979a95e59638884ae59d2af77523ced52805df944d", "container_config": { "AttachStderr": false, "AttachStdin": false, "AttachStdout": false, "Cmd": [ "/bin/sh", "-c", "#(nop) ENV STO=/media/data2/STO/els" ], "CpuShares": 0, "Cpuset": "", "Domainname": "", "Entrypoint": null, "Env": [ "PATH=/usr/lib/jvm/java-8-oracle/jre/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin", "JAVA_HOME=/usr/lib/jvm/java-8-oracle/jre", "ECLIPSE_HOME=/opt/eclipse/luna", "MAVEN_HOME=/usr/lib/apache/maven/3.2.5", "HADOOP_HOME=/usr/lib/apache/hadoop/2.6.0", "HADOOP_DATA_DIR=/home/craig/HADOOP_DATA_DIR", "HADOOP_CONF_DIR=/usr/lib/apache/hadoop/2.6.0/conf", "HADOOP_LOG_DIR=/usr/lib/apache/hadoop/2.6.0/logs", "WS=~/workspaces", "WSCOM=/home/craig/workspaces/swtk/common/projects/swtk-common", "WSSAN=/home/craig/workspaces/swtk/sandbox/projects/swtk-sandbox", "NYT=/media/data/NYT/02_Unzipped/02_TXT", "STO=/media/data2/STO/els" ], "ExposedPorts": null, "Hostname": "2338b11efbe1", "Image": "90dd9b6eab1d605a10e151660f2a841c4bc64978fb85d3424f97bdb40c94be28", "Memory": 0, "MemorySwap": 0, "NetworkDisabled": false, "OnBuild": [], "OpenStdin": false, "PortSpecs": null, "StdinOnce": false, "Tty": false, "User": "", "Volumes": null, "WorkingDir": "" }, "created": "2015-03-26T19:04:55.686342953Z", "docker_version": "1.2.0", "id": "73e5400f4bb13d4e12b0c457d8fb77603691e983254ba1a55b593140e9c096a6", "os": "linux", "parent": "90dd9b6eab1d605a10e151660f2a841c4bc64978fb85d3424f97bdb40c94be28" }

Note lines 16-17 that contain the modified PATH and environment variables

When I launch the container from this modified image, I get this:

craig@dockerhost:~/mydocker$ sudo docker run -i -t basehadoop /bin/bash
root@0dab6fe52a8e:/# echo $JAVA_HOME
/usr/lib/jvm/java-8-oracle/jre
root@0dab6fe52a8e:/# echo $PATH
/usr/lib/jvm/java-8-oracle/jre/bin:/usr/local/sbin:/usr/local/bin:/usr/sbin:/usr/bin:/sbin:/bin

References

1. [StackOverflow] How to update the PATH
2. Reference Dockerfiles:
1. [Github] Dockerfile for Java installation
2. [Github] sequenceiq/hadoop
3. [Blog] Removing untagged images from docker:
1. Remove all stopped containers:
   

   sudo docker rm $(sudo docker ps -a -q)
   

2. Remove all untagged images
   

   sudo docker rmi $(sudo docker images | grep "^<none>" | awk "{print $3}")