Image naming conventions (registry/image:tag) in Docker - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is the general format of a Docker image name?

A Docker image name usually follows this format: <registry>/<image>:<tag>. The registry is optional and defaults to Docker Hub if omitted.

beginner

What does the <tag> part in a Docker image name represent?

The <tag> specifies the version or variant of the image. If no tag is given, Docker uses the default tag 'latest'.

intermediate

Why include a registry in the Docker image name?

Including a registry tells Docker where to find the image. It can be Docker Hub or a private registry. This helps Docker pull the image from the correct source.

beginner

Example: What does 'myregistry.com/myapp/backend:1.2' mean?

It means the image is stored in 'myregistry.com' under the repository 'myapp/backend' with the version tag '1.2'.

beginner

What happens if you omit the tag in a Docker image name?

Docker assumes the tag 'latest' by default, so 'ubuntu' is the same as 'ubuntu:latest'.

What is the default tag used if none is specified in a Docker image name?

A 1.0

B stable

C default

D latest

If no tag is specified, Docker uses 'latest' as the default tag.

Which part of the Docker image name specifies where to find the image?

A tag

B registry

C image

D version

The registry part tells Docker which server or service to pull the image from.

In the image name 'nginx:alpine', what does 'alpine' represent?

A Tag

B Image name

C Registry

D Repository

'alpine' is the tag specifying a lightweight version of the nginx image.

What happens if you use 'docker pull ubuntu' without a tag?

A Pulls ubuntu:default

B Pulls ubuntu:stable

C Pulls ubuntu:latest

D Returns an error

Docker assumes the 'latest' tag if none is provided.

Which of these is a valid Docker image name?

A myregistry.com/myapp:1.0

B myapp:1.0:extra

C registry//image:tag

D image:tag:version

The format /: is valid. Multiple colons or double slashes are invalid.

Explain the parts of a Docker image name and why each part is important.

Think about where the image is stored, what it is called, and which version to use.

Describe what happens when you pull a Docker image without specifying a tag.

Consider what Docker assumes and why specifying a tag can be safer.

Practice

1. Which part of the Docker image name docker.io/library/nginx:latest specifies the version of the image?

easy

A. latest

B. docker.io

C. nginx

D. library

Solution

1. Step 1: Identify the tag in the image name

   The tag is the part after the colon (:), which specifies the version or variant of the image.

2. Step 2: Locate the tag in the example

   In docker.io/library/nginx:latest, the tag is latest.

3. Final Answer:

   latest -> Option A

4. Quick Check:

   Tag = latest [OK]

Hint: Tag is always after colon ':' in image name [OK]

Common Mistakes:

• Confusing registry with tag
• Thinking image name is the tag
• Ignoring the colon separator

2. Which of the following is the correct syntax for a Docker image name with a registry, image, and tag?

easy

A. tag/image:registry

B. registry/image:tag

C. image:tag/registry

D. image/registry:tag

Solution

1. Step 1: Recall the Docker image naming format

   The correct format is registry/image:tag, where registry is optional but comes first, then image name, then tag after colon.

2. Step 2: Match the options to the format

   Only registry/image:tag matches the correct order and separators.

3. Final Answer:

   registry/image:tag -> Option B

4. Quick Check:

   Format = registry/image:tag [OK]

Hint: Registry first, then image, then tag after ':' [OK]

Common Mistakes:

• Placing tag before image
• Swapping registry and image order
• Using wrong separators like '/' for tag

3. What is the full image name including registry and tag for the official Redis image version 7.0 from Docker Hub?

medium

A. docker.io/redis7:0

B. redis:7.0

C. redis/docker.io:7.0

D. docker.io/redis:7.0

Solution

1. Step 1: Understand Docker Hub default registry

   Docker Hub's registry is docker.io. The official images are under this registry.

2. Step 2: Compose full image name

   The full name includes registry, image name, and tag: docker.io/redis:7.0.

3. Final Answer:

   docker.io/redis:7.0 -> Option D

4. Quick Check:

   Full name = docker.io/redis:7.0 [OK]

Hint: Full name = registry/image:tag, Docker Hub registry is docker.io [OK]

Common Mistakes:

• Putting registry after image
• Missing registry for full name
• Incorrect tag format

4. You try to pull the image myregistry.com/myapp but get an error. Which of these is a likely cause?

medium

A. Missing tag defaults to 'latest', but image may not have it

B. Registry name cannot contain dots '.'

C. Image name must always include a tag

D. Registry must be 'docker.io' only

Solution

1. Step 1: Understand default tag behavior

   If no tag is given, Docker uses latest by default. If the image does not have a 'latest' tag, pull fails.

2. Step 2: Check other options for errors

   Registry names can contain dots, tags are optional, and registries can be other than docker.io.

3. Final Answer:

   Missing tag defaults to 'latest', but image may not have it -> Option A

4. Quick Check:

   Default tag 'latest' may be missing [OK]

Hint: No tag means 'latest' assumed, may cause pull failure [OK]

Common Mistakes:

• Thinking registry can't have dots
• Believing tag is mandatory
• Assuming only docker.io registry works

5. You want to tag a local image myapp to push it to a private registry at registry.example.com with version v2. Which command correctly tags the image?

hard

A. docker tag myapp:v2 registry.example.com/myapp

B. docker tag registry.example.com/myapp:v2 myapp

C. docker tag myapp registry.example.com/myapp:v2

D. docker tag myapp registry.example.com:v2/myapp

Solution

1. Step 1: Understand docker tag syntax

   The syntax is docker tag SOURCE_IMAGE[:TAG] TARGET_IMAGE[:TAG].

2. Step 2: Apply to given names

   Source is myapp (local), target is registry.example.com/myapp:v2 to specify registry and tag.

3. Final Answer:

   docker tag myapp registry.example.com/myapp:v2 -> Option C

4. Quick Check:

   Tag command: source then target with registry and tag [OK]

Hint: Tag command: source image then target with registry:tag [OK]

Common Mistakes:

• Swapping source and target images
• Placing tag in wrong position
• Using invalid registry/tag format