Pushing images to registry in Docker - Step-by-Step Execution

Process Flow - Pushing images to registry

Build Docker Image

↓

Tag Image with Registry URL

↓

Login to Registry

↓

Push Image to Registry

↓

Verify Image in Registry

↓

Done

This flow shows the steps to build a Docker image, tag it for a registry, login, push it, and verify the push.

Execution Sample

Docker

docker build -t myapp:1.0 .
docker tag myapp:1.0 myregistry.com/myapp:1.0
docker login myregistry.com
docker push myregistry.com/myapp:1.0

Builds an image, tags it with registry URL, logs in, then pushes the image to the registry.

Process Table

Step	Command	Action	Result	Notes
1	docker build -t myapp:1.0 .	Build image from Dockerfile	Image 'myapp:1.0' created locally	Image ready for tagging
2	docker tag myapp:1.0 myregistry.com/myapp:1.0	Tag image with registry URL	Image tagged as 'myregistry.com/myapp:1.0'	Tag links local image to remote name
3	docker login myregistry.com	Authenticate to registry	Login successful	Allows pushing images
4	docker push myregistry.com/myapp:1.0	Push image to registry	Image uploaded to registry	Image now available remotely
5	Verify image in registry	Check registry for image	Image found with tag '1.0'	Push confirmed successful

💡 Image pushed and verified in registry, process complete

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 4	Final
Image Name	none	myapp:1.0	myregistry.com/myapp:1.0	myregistry.com/myapp:1.0	myregistry.com/myapp:1.0
Registry Login Status	not logged in	not logged in	not logged in	logged in	logged in
Image Location	none	local	local	remote and local	remote and local

Key Moments - 3 Insights

Why do we need to tag the image before pushing?

Tagging links the local image to the remote registry name. Without tagging, Docker doesn't know where to push the image. See execution_table step 2.

What happens if we try to push without logging in?

The push will fail because the registry requires authentication. Step 3 shows login is needed before pushing in step 4.

Does pushing remove the local image?

No, pushing uploads a copy to the registry but keeps the local image intact. Variable 'Image Location' shows it remains local and remote after push.

Visual Quiz - 3 Questions

Look at the execution table, what is the image name after tagging (step 2)?

A myregistry.com/myapp:1.0

B myapp:1.0

C myregistry.com:1.0

D myapp

💡 Hint

Check the 'Result' column in step 2 of the execution_table.

At which step does the user authenticate to the registry?

A Step 1

B Step 2

C Step 3

D Step 4

💡 Hint

Look for the 'Action' column mentioning authentication in the execution_table.

If the login step is skipped, what will happen at the push step?

A Push succeeds without issues

B Push fails due to authentication error

C Image is pushed but not tagged

D Docker automatically logs in

💡 Hint

Refer to key_moments explanation about login necessity before push.

Concept Snapshot

Docker image push steps:
1. Build image locally with 'docker build -t name:tag .'
2. Tag image with registry URL using 'docker tag'
3. Login to registry with 'docker login'
4. Push image using 'docker push'
5. Verify image is in registry
Tagging links local image to remote name; login is required before push.

Full Transcript

To push a Docker image to a registry, first build the image locally using 'docker build'. Then tag the image with the registry URL so Docker knows where to send it. Next, login to the registry to authenticate. After successful login, push the tagged image to the registry. Finally, verify the image is available remotely. Tagging is essential to link the image to the registry path. Login is required to allow pushing. Pushing uploads the image but keeps the local copy intact.

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