Content trust and image signing in Docker - Step-by-Step Execution

Process Flow - Content trust and image signing

Build Docker Image

↓

Sign Image with Notary

↓

Push Signed Image to Registry

↓

Enable Content Trust on Client

↓

Pull Image

↓

Verify Signature

Yes No↓

Run Image

This flow shows how a Docker image is built, signed, pushed, and then verified on pull before running.

Execution Sample

Docker

export DOCKER_CONTENT_TRUST=1

docker build -t myapp:1.0 .
docker push myapp:1.0
docker pull myapp:1.0

This sequence builds an image, pushes it with a signature, then pulls and verifies the signature before use.

Process Table

Step	Command	Action	Result	Notes
1	docker build -t myapp:1.0 .	Build image locally	Image 'myapp:1.0' created	Image ready for signing
2	docker push myapp:1.0	Push image and sign	Image and signature uploaded	Signature stored in Notary server
3	export DOCKER_CONTENT_TRUST=1	Enable content trust	Client will verify signatures	Prevents unsigned image pull
4	docker pull myapp:1.0	Pull image with verification	Image pulled and signature verified	Image trusted and safe to run
5	docker pull unsignedimage:latest	Pull unsigned image with trust enabled	Pull fails with error	Prevents running untrusted images

💡 Content trust enabled, unsigned images are rejected on pull

Status Tracker

Variable	Start	After Step 1	After Step 2	After Step 3	After Step 4	After Step 5
DOCKER_CONTENT_TRUST	0 (disabled)	0	0	1 (enabled)	1	1
Image 'myapp:1.0'	none	built	pushed & signed	pushed & signed	pulled & verified	pulled & verified
Image 'unsignedimage:latest'	none	none	none	none	none	pull failed

Key Moments - 3 Insights

Why does the pull fail when content trust is enabled for an unsigned image?

Because with content trust enabled (see step 3 and 5 in execution_table), Docker requires a valid signature. Without it, the pull is rejected to keep the system safe.

What happens if you push an image without signing it?

The push will succeed but without a signature. Later, if content trust is enabled, pulling that image will fail (see step 5). Signing happens during push if trust is enabled.

Does enabling content trust affect building images?

No, building images is unaffected (step 1). Content trust only affects pushing and pulling images by requiring signatures.

Visual Quiz - 3 Questions

Look at the execution_table, what is the state of DOCKER_CONTENT_TRUST after step 3?

A Enabled (1)

B Unset

C Disabled (0)

D Partially enabled

💡 Hint

Check variable_tracker column 'After Step 3' for DOCKER_CONTENT_TRUST

At which step does the image get signed?

A Step 1 - build

B Step 2 - push

C Step 3 - enable trust

D Step 4 - pull

💡 Hint

Look at execution_table 'Action' and 'Result' columns for step 2

If DOCKER_CONTENT_TRUST was not enabled, what would happen at step 5 when pulling an unsigned image?

A Image gets signed automatically

B Pull fails with error

C Pull succeeds without verification

D Docker blocks all pulls

💡 Hint

Refer to variable_tracker and execution_table step 5 for pull behavior with trust enabled

Concept Snapshot

Content trust in Docker ensures images are signed and verified.
Enable with DOCKER_CONTENT_TRUST=1.
Sign images during push; verify on pull.
Unsigned images are rejected if trust is enabled.
Protects from running untrusted code.

Full Transcript

This visual execution shows how Docker content trust works. First, you build an image locally. Then you push it to a registry, where it gets signed using Notary. On the client side, you enable content trust by setting the environment variable DOCKER_CONTENT_TRUST=1. When you pull the image, Docker checks the signature. If the signature is valid, the image is pulled and can be run safely. If the image is unsigned and content trust is enabled, the pull fails to prevent running untrusted images. This process helps keep your Docker environment secure by verifying image authenticity.

Practice

1. What is the main purpose of Docker content trust?

easy

A. To speed up Docker image downloads

B. To verify the authenticity and integrity of Docker images

C. To compress Docker images for storage

D. To automatically update Docker images

Solution

1. Step 1: Understand content trust concept

   Content trust in Docker is designed to ensure images are verified and safe to use by checking their signatures.

2. Step 2: Identify the purpose from options

   Only To verify the authenticity and integrity of Docker images mentions verifying authenticity and integrity, which matches content trust's goal.

3. Final Answer:

   To verify the authenticity and integrity of Docker images -> Option B

4. Quick Check:

   Content trust = verify image authenticity [OK]

Hint: Content trust means verifying image signatures before use [OK]

Common Mistakes:

• Confusing content trust with image compression
• Thinking content trust speeds up downloads
• Assuming content trust auto-updates images

2. Which command correctly enables Docker content trust before pulling an image?

easy

A. set DOCKER_CONTENT_TRUST=1 docker pull alpine

B. docker pull --trust alpine

C. export DOCKER_CONTENT_TRUST=1 && docker pull alpine

D. docker pull alpine --enable-content-trust

Solution

1. Step 1: Recall how to enable content trust

   Content trust is enabled by setting the environment variable DOCKER_CONTENT_TRUST=1 before running docker pull.

2. Step 2: Match correct syntax

   export DOCKER_CONTENT_TRUST=1 && docker pull alpine uses the correct shell syntax to export the variable and then pull the image.

3. Final Answer:

   export DOCKER_CONTENT_TRUST=1 && docker pull alpine -> Option C

4. Quick Check:

   Set env var DOCKER_CONTENT_TRUST=1 before pull [OK]

Hint: Use export DOCKER_CONTENT_TRUST=1 before docker pull [OK]

Common Mistakes:

• Using incorrect flags like --trust or --enable-content-trust
• Trying to set env var inline without export
• Confusing Windows and Linux environment variable syntax

3. What will happen if you run docker pull alpine with DOCKER_CONTENT_TRUST=1 set but the image is unsigned?

medium

A. The image will be pulled without verification

B. Docker will automatically sign the image locally

C. Docker will pull the image but warn about missing signature

D. Docker will refuse to pull the image and show an error

Solution

1. Step 1: Understand content trust behavior on unsigned images

   When content trust is enabled, Docker requires a valid signature to pull an image.

2. Step 2: Identify expected Docker response

   If the image is unsigned, Docker refuses to pull it and shows an error to prevent unverified usage.

3. Final Answer:

   Docker will refuse to pull the image and show an error -> Option D

4. Quick Check:

   Content trust blocks unsigned image pulls [OK]

Hint: Content trust blocks unsigned images from pulling [OK]

Common Mistakes:

• Thinking Docker pulls unsigned images anyway
• Assuming Docker signs images automatically
• Believing Docker only warns but allows pull

4. You enabled content trust but get an error: no trust data available when pulling an image. What is the likely cause?

medium

A. The image has no signed tags in the registry

B. Docker daemon is not running

C. You forgot to set DOCKER_CONTENT_TRUST=0

D. Your internet connection is down

Solution

1. Step 1: Analyze the error message meaning

   'No trust data available' means Docker cannot find signed metadata for the image tags in the registry.

2. Step 2: Match cause to options

   The image has no signed tags in the registry correctly states the image lacks signed tags, causing this error when content trust is enabled.

3. Final Answer:

   The image has no signed tags in the registry -> Option A

4. Quick Check:

   No trust data = no signed tags in registry [OK]

Hint: No trust data means image tags are unsigned [OK]

Common Mistakes:

• Assuming Docker daemon stopped causes this error
• Thinking DOCKER_CONTENT_TRUST=0 fixes error when trust is needed
• Blaming internet connection without checking image signing

5. You want to ensure only signed images are deployed in your production environment. Which approach best achieves this?

hard

A. Set DOCKER_CONTENT_TRUST=1 on all production servers and pull images normally

B. Manually verify image signatures after pulling unsigned images

C. Disable content trust and rely on image tags naming conventions

D. Use docker pull --no-trust to skip signature checks

Solution

1. Step 1: Identify how to enforce signed image usage

   Setting DOCKER_CONTENT_TRUST=1 forces Docker to only pull images with valid signatures, blocking unsigned ones.

2. Step 2: Evaluate options for production safety

   Set DOCKER_CONTENT_TRUST=1 on all production servers and pull images normally correctly enforces signature verification automatically on production servers.

3. Final Answer:

   Set DOCKER_CONTENT_TRUST=1 on all production servers and pull images normally -> Option A

4. Quick Check:

   Enable content trust to enforce signed images [OK]

Hint: Enable DOCKER_CONTENT_TRUST=1 in production for safety [OK]

Common Mistakes:

• Trying to verify signatures manually after pull
• Disabling content trust and trusting tags only
• Using flags to skip trust checks in production