Pushing images to registry in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the purpose of pushing a Docker image to a registry?

Pushing a Docker image to a registry stores the image in a central place so others can download and use it on their machines or servers.

beginner

Which command tags a local Docker image before pushing it to a registry?

The command is docker tag <local-image> <registry-url>/<repository>:<tag>. This prepares the image with the correct name for the registry.

beginner

What command do you use to push a Docker image to a registry?

Use docker push <registry-url>/<repository>:<tag> to upload the image to the registry.

beginner

Why do you need to log in to a Docker registry before pushing images?

Logging in with docker login lets the registry know who you are and if you have permission to upload images.

intermediate

What happens if you try to push an image without tagging it with the registry URL?

Docker will try to push to the default Docker Hub repository. If the image name is not recognized there, the push will fail.

Which command tags a Docker image for pushing to a registry?

A docker build myimage

B docker tag myimage myregistry.com/myrepo:latest

C docker login myregistry.com

D docker push myimage

Tagging prepares the image with the registry URL and repository name before pushing.

What is the correct command to upload an image to a Docker registry?

A docker tag myimage myrepo:latest

B docker pull myregistry.com/myrepo:latest

C docker push myregistry.com/myrepo:latest

D docker run myregistry.com/myrepo:latest

The 'docker push' command uploads the image to the specified registry.

Why do you run 'docker login' before pushing an image?

A To authenticate and get permission to push images

B To download images from the registry

C To build the Docker image

D To remove old images

'docker login' authenticates your user to the registry for pushing images.

If you push an image without tagging it with a registry URL, where does Docker try to push it?

A Docker Hub (default public registry)

B Local machine only

C Private registry automatically

D It does not push anywhere

Docker defaults to Docker Hub if no registry URL is specified.

What must you do before pushing a newly built Docker image?

A Pull the image from the registry

B Run the image locally

C Delete old images

D Tag the image with the registry and repository name

Tagging the image with the registry URL is required to push it correctly.

Explain the steps to push a Docker image to a remote registry.

Think about preparing the image, authenticating, and then uploading.

Why is tagging important before pushing a Docker image to a registry?

Consider how Docker knows where to send the image.

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