How to Secure Your Docker Images: 3 Essential Practices

Is Your Image Secure?

Compared with traditional servers and VMs, Docker containers provide a safer environment by isolating application components, reducing the attack surface, and limiting the impact of a breach.

Nevertheless, understanding Docker’s own security risks is essential to protect containerized systems.

Run Container Images as Non‑Root Users

By default Docker grants root privileges to processes inside a container, which gives full control over the container and host. Deploying containers as non‑root users prevents attackers from exploiting root access to steal credentials or tamper with the host.

How to Prevent Containers from Running as Root

If the base image’s user is unknown, enforce a custom non‑root user or group so the container can only access required resources.

Set a non‑root user in the Dockerfile

First, create a dedicated user or group with only the permissions needed by the application, then add a USER instruction in the Dockerfile.

FROM centos:7

RUN groupadd -g 1000 basicuser && \
    useradd -r -u 1000 -g basicuser basicuser
USER basicuser

Specify a non‑root user with --user in docker run The --user flag overrides any user set in the Dockerfile, ensuring the container runs with the lowest privileges (e.g., UID 1009). However, it does not fix security flaws in the image itself, so defining a non‑root user in the Dockerfile is recommended.

$ docker run --user 1009 centos:7

Use Your Own Private Registry

A private registry is an isolated image repository you host yourself, either on-premises or on cloud services such as Amazon ECR, Azure Container Registry, Google Container Registry, Red Hat Quay, or JFrog.

Private registries offer advanced features that improve image security, including:

Comprehensive image scanning for vulnerabilities

Strict governance with role‑based access control (RBAC) and compliance monitoring

Digital signatures, image authentication, and tamper‑proofing

Multi‑environment repositories for development, testing, and production

Public registries like Docker Hub often provide only basic services, requiring you to trust the image publisher, which can expose you to malicious or outdated images.

Keep Images Minimal

Larger images increase the attack surface. For Docker, you can choose only the components you need.

Choose the Smallest Base Image

Some images on Docker Hub are significantly larger than others. Prefer the smallest base image that satisfies your requirements and add only the necessary packages.

After pulling an image, use docker images to inspect its actual size.

$ docker images

Optimize Dockerfile and .dockerignore

Create a Dockerfile that builds a streamlined image, separating the base layer from your own layers. Use a .dockerignore file to exclude files that are not needed at runtime.

Multi‑Stage Builds

Docker’s multi‑stage build feature (available from Docker 17.05) lets you use multiple FROM statements, copying only the necessary artifacts to the final stage, which reduces image size.

FROM golang:1.7.3

WORKDIR /go/src/github.com/alexellis/href-counter/
RUN go get -d -v golang.org/x/net/html
COPY app.go .
RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o app .

FROM alpine:latest
RUN apk --no-cache add ca-certificates
WORKDIR /root/
COPY --from=0 /go/src/github.com/alexellis/href-counter/app .
CMD ["./app"]

Verify Image Integrity

Docker Content Trust, introduced in Docker Engine 1.8, enables digital signing and verification of images. When enabled, Docker automatically validates the signature of pulled images, confirming the publisher’s identity.

export DOCKER_CONTENT_TRUST=1

Setting this variable in the current shell activates trust for that session; to enable it system‑wide, configure the variable in a shared environment file.

Article source: JFrog DevOps