Unsigned images won’t run when Docker Content Trust is enabled.

Outline (skeleton)

• Warm introduction: what Docker Content Trust (DCT) is and why it matters

• The question in plain speech: which statement truly reflects DCT? the right answer and what it means

• How DCT works, in everyday terms: signing, verification, and the “bouncer” metaphor

• Why unsigned images can’t run with DCT enabled, and what that buys you

• Debunking the common myths (the other answer choices) with quick explanations

• Getting practical: how to enable DCT, sign images, and verify them

• Real‑world takeaways: security, reliability, and peace of mind

• Quick wrap‑up: what to remember about DCT and Docker images

Docker Content Trust: a practical guide you can feel in your workflow

If you’ve ever waited for a trusted voice to validate something before you click “run,” you’ll understand Docker Content Trust (DCT) at a gut level. DCT is a security feature in Docker that makes sure images you pull and run are signed and verified. Think of it as a digital stamp of authenticity, a kind of curbside check that says, “Yep, this image came from the right people and hasn’t been tampered with.”

Which statement actually reflects DCT?

Here’s the key idea, explained plainly: when DCT is on, unsigned images won’t run. The simple truth is B: unsigned images will not run when DCT is enabled. That’s the guardrail. It’s not about restricting pulling only or about requiring every image to be “verified individually” in isolation; it’s about the whole image lifecycle: it must be signed and validated before you can even start it. If the image doesn’t carry a trusted signature, Docker stops you at the door.

A quick, human-friendly way to think about it: you wouldn’t go into a club without ID and a bouncer wouldn’t let you in if your badge wasn’t valid. DCT is that badge system for container images. It’s not trying to be punitive; it’s designed to prevent bad copies from running in your environment, which can save you from a lot of headaches later on.

What exactly does DCT do, and why does it matter?

• Signing is the act of attaching a digital signature to an image’s metadata. This signature proves who built the image and that its contents haven’t been altered since it was signed.

• Verification checks that signature against a trusted signing authority (historically Notary, among others). If the signature checks out, Docker allows the image to be pulled and run; if not, it blocks the action.

• The policy is enforced by enabling DCT in your Docker client. Once active, Docker will only accept images that have a valid signature from a trusted source.

Let me explain the “bouncer at the club” analogy a bit more. Your registry is like a venue with a strict guest list. The signing authority is the host who signs off on each guest (image). When you attempt to pull or run an image, the system checks your guest certificate against the list. If something isn’t signed, or the signature doesn’t match, you don’t get in. No questions asked. This is the core of why unsigned images aren’t allowed when DCT is enabled.

Common myths, cleared up

• A says: All images can run regardless of their signatures. Not true when DCT is active. The signature check is part of the gatekeeping.

• C says: Only signed images can be pulled but not run. That’s a misreading. With DCT, both pulling and running can be blocked if the image isn’t signed or fails verification.

• D says: All trusted images must be verified individually. The reality is a bit more nuanced. Verification happens as part of the trust framework, but the system’s main job is ensuring only signed, trusted images are used. It’s not about a manual, per-image process every single time, but about a policy that enforces trust consistently.

If you’re mapping this to real-life workflows, think about the entire supply chain. You don’t want to rely on a signature here and a verification there; you want a consistent, repeatable trust process. DCT gives you that consistency.

From theory to practice: enabling DCT and working with signed images

Getting DCT up and running is pretty approachable for teams that already have a signing workflow. Here are the high-level steps you’ll encounter in a typical, modern Docker setup:

• Enable DCT in your environment. Practically, that means setting up your Docker client to require content trust. In many environments, you set DOCKER_CONTENT_TRUST=1 in your shell so the client enforces signatures on pull and run.

• Sign your images. This is where the signing authority comes into play. You’ll sign images before sharing them in a registry. The signing step creates that digital stamp that the verifier will later check.

• Push signed images to a trusted registry. The registry holds the signed artifacts and their corresponding signatures so that any pulling client can verify them.

• Verify during pull/run. If an image is signed by a trusted authority and the signature is intact, Docker proceeds. If not, you’ll see a blocked pull or run with a clear message about the missing or invalid signature.

• Maintain rotation and revocation. Like any security policy, you’ll want to manage signing keys, rotate them periodically, and revoke any compromised signatures if needed.

If you’ve worked with a CI/CD pipeline, you’ll find DCT fits neatly into builds and deployments. Build a trusted image, sign it automatically as part of the pipeline, push it to a registry, and have your deployment stage pull only signed images. It’s a clean, auditable flow that pays off when you’re juggling multiple teams and environments.

Practical notes and gentle caveats

• Not all registries support every flavor of signing. Some teams start with Docker Content Trust using Notary, while others explore newer signing tools that integrate with the registry’s trust policies. The key is choosing a signing and verification approach that fits your registry and your security posture.

• Enabling DCT isn’t a silver bullet. You still need good key management, rotation policies, and clear ownership over signing. It’s part of a broader approach to software supply chain security.

• In development environments, you might choose to relax DCT temporarily for speed while you’re iterating. In production, the expectation is to keep DCT enabled and bindings tight.

A quick, creator-friendly checklist you can keep on hand

• Turn on DCT in your Docker client (set DOCKER_CONTENT_TRUST=1 as a baseline).

• Sign every image as part of your build process.

• Push signed images to a registry you control or trust, with proper access controls.

• Verify images during pull/run using the registry’s signing information.

• Regularly review signing keys, rotation schedules, and revocation lists.

• Document your trust policy so the whole team knows how to handle images from signing to deployment.

• Monitor for failed verifications and set up alerts so you can respond quickly.

Real-world reflections: why this matters, beyond the buzzwords

Security often feels abstract until you’ve seen the consequences of a compromised image. Imagine an attacker injecting a malicious layer into a base image you trust. If your chain of trust isn’t solid, that bad actor’s code could slip into production, quietly masquerading as a trusted artifact. DCT isn’t about fear; it’s about reducing risk in a practical, measurable way. When unsigned images are blocked, you’re not just enforcing a rule—you’re elevating the reliability of your services and the confidence of your team.

A few more thoughts that tend to resonate

• Design and governance go hand in hand with DCT. The best setups pair strong signing workflows with clear ownership and audit trails. It’s easier to move fast when you’re not chasing after “unknown” images in the middle of a critical deployment.

• The human side matters. Automate signing where possible, but also cultivate a culture of caution around third-party images. Trust is earned, not assumed.

• The tech evolves, and so do signing methods. Stay curious about updates to signing tools, registry features, and verification strategies, because a small improvement today can save you a lot of headaches tomorrow.

Wrapping it up with a simple takeaway

When DCT is enabled, unsigned images don’t get to run. That’s the guardrail in action—a straightforward rule with a big impact on security and reliability. If you’re building modern, containerized applications, embracing this discipline helps you ship with a clearer sense of who you trust and where those images came from. It’s not about paranoia; it’s about making your environments safer, more predictable, and easier to manage as teams grow and projects scale.

If you want a mental shorthand: think of DCT as Docker’s trust badge and the edgy, friendly bouncer that keeps the club honest. With it in place, you know exactly what you’re running, where it came from, and that it’s been checked against a trusted source. That clarity is worth more than a moment’s hesitation when you’re deploying critical services.

And that’s the spirit of Docker Content Trust—practical security that fits naturally into real-world workflows, without adding noise or friction to your day.