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Overriding Environment Variables

Docker Documentation References:

Dockerfile

docker build

docker run

docker images

docker rmi

Intent

The purpose of this kata is to familiarize yourself with the process of overriding an environment variable that was set in the Dockerfile.

Overview

In this exercise we will create a simple Dockerfile and set an environment variable with will impact the output of the application. We will then delete the image.

Kata Steps

Create a Dockerfile

Use and editor to create a file named sh_hello_user_Dockerfile, or copy the sh_hello_user_Dockerfile from this repository.

The file should contain the following

FROM alpine:3.6
MAINTAINER [email protected]

ENV DOCKER_USER_NAME Timmy

ENTRYPOINT [ "sh", "-c", "echo $DOCKER_USER_NAME" ]

Build the IMage

Command

docker build -f sh_hello_user_Dockerfile .

Output

thought:DockerKata rich$ docker build -f sh_hello_user_Dockerfile .
Sending build context to Docker daemon 434.2 kB
Step 1/4 : FROM alpine:3.6
 ---> a41a7446062d
Step 2/4 : MAINTAINER [email protected]
 ---> Running in a78b6083f640
 ---> dc97dfd78be5
Removing intermediate container a78b6083f640
Step 3/4 : ENV DOCKER_USER_NAME Timmy
 ---> Running in e52461bfa94a
 ---> 56d9cb50cc30
Removing intermediate container e52461bfa94a
Step 4/4 : ENTRYPOINT sh -c echo $DOCKER_USER_NAME
 ---> Running in 72f900e1536b
 ---> 804eb58d237f
Removing intermediate container 72f900e1536b
Successfully built 804eb58d237f

List Images

Command

docker images

Output

thought:DockerKata rich$ docker images
REPOSITORY                                   TAG                 IMAGE ID            CREATED             SIZE
<none>                                       <none>              804eb58d237f        3 seconds ago       3.97 MB

Note: We will use the IMAGE ID which will differ from machine to machine, so you must select what is on your computer for the next two steps.

Run the Image

Command

docker run --env DOCKER_USER_NAME=$USER 804eb58d237f

Output

thought:DockerKata rich$ docker run --env DOCKER_USER_NAME=$USER 804eb58d237f
rich

Delete the Image (and related container)

Command

docker rmi -f 804eb58d237f

Output

thought:DockerKata rich$ docker rmi -f 804eb58d237f
Deleted: sha256:804eb58d237fb2b1dbe66d548779419825aaff1bd7b1d42c0216ec596569e846
Deleted: sha256:56d9cb50cc306cd2ce5a130bb99948a51d8261572a336b605a1304b74269ce1e
Deleted: sha256:dc97dfd78be5133d39a36e9962fbe197e92744c373684a1cbf295ff3afdaec69

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