Pushing images to registry in Docker - Commands & Configuration

Introduction

When you build a container image on your computer, it stays local. To share it with others or use it on servers, you need to upload it to a central place called a registry. Pushing images to a registry lets you store and share your container images easily.

When you want to share your app's container image with your team.

When you need to deploy your container image to a cloud server.

When you want to keep a backup of your container images online.

When you want to use the same container image on multiple machines.

When you want to automate deployments using images stored in a registry.

Commands

This command builds a container image from the current folder and tags it as 'my-app' with version '1.0'. Tagging helps identify the image later.

Terminal

docker build -t my-app:1.0 .

Expected Output

Sending build context to Docker daemon  2.048kB

Step 1/3 : FROM alpine

 ---> a24bb4013296

Step 2/3 : RUN echo "Hello from my app"

 ---> Running in 123abc456def

Removing intermediate container 123abc456def

 ---> 789def012ghi

Step 3/3 : CMD ["echo", "Hello from my app"]

 ---> Running in 456ghi789jkl

Removing intermediate container 456ghi789jkl

 ---> 345jkl678mno

Successfully built 345jkl678mno

Successfully tagged my-app:1.0

→

-t - Tags the image with a name and version

This command tags your local image with the registry address so Docker knows where to push it.

Terminal

docker tag my-app:1.0 myregistry.example.com/my-app:1.0

Expected Output

No output (command runs silently)

This command logs you into the registry so you have permission to upload images. It will ask for your username and password.

Terminal

docker login myregistry.example.com

Expected Output

Username: user123

Password: 

Login Succeeded

This command uploads your tagged image to the registry so others can download and use it.

Terminal

docker push myregistry.example.com/my-app:1.0

Expected Output

The push refers to repository [myregistry.example.com/my-app]

1.0: Pushed

Digest: sha256:abcdef1234567890abcdef1234567890abcdef1234567890abcdef1234567890

Status: Image is up to date for myregistry.example.com/my-app:1.0

This command lists all local images so you can verify your image is built and tagged correctly.

Terminal

docker images

Expected Output

REPOSITORY                          TAG       IMAGE ID       CREATED          SIZE

my-app                             1.0       345jkl678mno   2 minutes ago    5MB

myregistry.example.com/my-app      1.0       345jkl678mno   2 minutes ago    5MB

Key Concept

If you remember nothing else from this pattern, remember: tag your image with the registry address before pushing it so Docker knows where to upload.

Common Mistakes

Trying to push an image without tagging it with the registry address.

Docker does not know where to send the image, so the push fails.

Use 'docker tag' to add the registry address to your image before pushing.

Not logging into the registry before pushing.

The registry denies permission, so the push is rejected.

Run 'docker login' with the registry URL and enter your credentials first.

Using 'latest' tag unintentionally when pushing.

It can cause confusion about which version is deployed or used.

Always tag images with explicit version numbers like '1.0' to avoid ambiguity.

Summary

Build your container image locally and tag it with a name and version.

Tag the image again with the registry address to prepare for upload.

Log in to the registry to authenticate your push.

Push the tagged image to the registry so it can be shared or deployed.

Verify your images locally to confirm tagging and building.

Practice

1.

What is the main purpose of the docker push command?

easy

A. Remove a Docker image from local storage

B. Download an image from a remote registry

C. Run a container from an image

D. Upload a local Docker image to a remote registry

Solution

1. Step 1: Understand the command purpose

   The docker push command is used to send a local image to a remote registry so others can access it.

2. Step 2: Compare options with command function

   Downloading images uses docker pull, running containers uses docker run, and removing images uses docker rmi. Only uploading matches docker push.

3. Final Answer:

   Upload a local Docker image to a remote registry -> Option D

4. Quick Check:

   Push means upload image [OK]

Hint: Push means upload your image to share it [OK]

Common Mistakes:

• Confusing push with pull (download)
• Thinking push runs containers
• Assuming push deletes images

2.

Which of the following is the correct syntax to push a Docker image named myapp with tag v1 to Docker Hub?

docker push ________

easy

A. myapp:v1

B. myapp

C. myapp:v1 latest

D. push myapp:v1

Solution

1. Step 1: Recall push command syntax

   The correct syntax is docker push [repository]:[tag]. The tag is required if not using default 'latest'.

2. Step 2: Analyze options

   myapp:v1 uses myapp:v1 which is correct. myapp misses the tag. myapp:v1 latest has extra word 'latest' which is invalid. push myapp:v1 repeats 'push' which is wrong.

3. Final Answer:

   myapp:v1 -> Option A

4. Quick Check:

   Push needs image name and tag [OK]

Hint: Use image name with tag like repo:tag [OK]

Common Mistakes:

• Omitting the tag when required
• Adding extra words after image name
• Repeating the push command inside

3.

Given the commands below, what will be the output of the docker push command?

docker tag myapp:latest myrepo/myapp:v2
docker push myrepo/myapp:v2

medium

A. Image pushed successfully to myrepo/myapp:v2

B. Error: image not found locally

C. Authentication required before push

D. Image pushed but with tag 'latest'

Solution

1. Step 1: Understand tagging and pushing

   The image myapp:latest is tagged as myrepo/myapp:v2, preparing it for push to myrepo registry.

2. Step 2: Predict push output

   Since the image is tagged correctly and assuming user is logged in, the push will succeed with a success message.

3. Final Answer:

   Image pushed successfully to myrepo/myapp:v2 -> Option A

4. Quick Check:

   Tag then push means success [OK]

Hint: Tag image with repo before push [OK]

Common Mistakes:

• Forgetting to tag image before push
• Not logging in to registry
• Assuming default tag is always 'latest'

4.

What is wrong with the following commands if the push fails?

docker push myapp:v1

Assuming myapp:v1 is a local image and the user wants to push to Docker Hub.

medium

A. The push command syntax is incorrect

B. Image is not tagged with the Docker Hub repository name

C. Docker daemon is not running

D. The image tag 'v1' is invalid

Solution

1. Step 1: Check image tagging for remote push

   To push to Docker Hub, the image must be tagged with the repository name like username/myapp:v1.

2. Step 2: Identify the cause of failure

   Since the image is only myapp:v1 without repo prefix, Docker Hub rejects the push.

3. Final Answer:

   Image is not tagged with the Docker Hub repository name -> Option B

4. Quick Check:

   Push needs repo prefix [OK]

Hint: Tag image with repo name before pushing [OK]

Common Mistakes:

• Trying to push without repo prefix
• Assuming push syntax is wrong
• Ignoring Docker daemon status

5.

You want to push a local image webapp:latest to a private registry at registry.example.com. Which sequence of commands is correct?

hard

A. docker login registry.example.comdocker push webapp:latest

B. docker push webapp:latestdocker login registry.example.com

C. docker login registry.example.comdocker tag webapp:latest registry.example.com/webapp:latestdocker push registry.example.com/webapp:latest

D. docker tag webapp:latest registry.example.com/webappdocker push webapp:latest

Solution

1. Step 1: Login to the private registry

   You must log in first to authenticate with registry.example.com.

2. Step 2: Tag the image with the registry URL

   Tag webapp:latest as registry.example.com/webapp:latest so Docker knows where to push.

3. Step 3: Push the tagged image to the registry

   Push the correctly tagged image to upload it to the private registry.

4. Final Answer:

   docker login registry.example.com docker tag webapp:latest registry.example.com/webapp:latest docker push registry.example.com/webapp:latest -> Option C

5. Quick Check:

   Login, tag with registry, then push [OK]

Hint: Login, tag with registry URL, then push [OK]

Common Mistakes:

• Pushing before login
• Not tagging with registry URL
• Pushing wrong image name