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guides: add docker+zscaler guide
Signed-off-by: David Karlsson <[email protected]>
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‎_vale/config/vocabularies/Docker/accept.txt‎

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OTel
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Okta
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PAT
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PEM
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Postgres
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PowerShell
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Python
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Wasm
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Windows
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WireMock
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Zscaler
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[Cc]odenames?

‎content/guides/zscaler/index.md‎

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---
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title: Using Docker with Zscaler
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tags: [networking, admin]
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summary: |
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This guide explains how to embed Zscaler’s root certificate into Docker
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images, allowing containers to operate securely with Zscaler proxies and
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avoid SSL errors.
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params:
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time: 10 minutes
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---
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In many corporate environments, network traffic is intercepted and monitored
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using HTTPS proxies, such as Zscaler. While Zscaler ensures security compliance
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and network control, it can cause issues for developers using Docker,
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particularly during build processes, where SSL certificate validation errors
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might occur. This guide outlines how to configure Docker containers and builds
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to properly handle Zscaler's custom certificates, ensuring smooth operation in
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monitored environments.
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## The role of certificates in Docker
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When Docker builds or runs containers, it often needs to fetch resources from
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the internet—whether it's pulling a base image from a registry, downloading
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dependencies, or communicating with external services. In a proxied
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environment, Zscaler intercepts HTTPS traffic and replaces the remote server's
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certificate with its own. However, Docker doesn't trust this Zscaler
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certificate by default, leading to SSL errors.
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```plaintext
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x509: certificate signed by unknown authority
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```
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These errors occur because Docker cannot verify the validity of the certificate
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presented by Zscaler. To avoid this, you must configure Docker to trust
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Zscaler's certificate.
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## Configure Zscaler proxy for Docker Desktop
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Depending on how Zscaler is deployed, you may need to configure Docker Desktop
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proxy settings manually to use the Zscaler proxy.
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If you're using Zscaler as a system-level proxy via the [Zscaler Client Connector](https://help.zscaler.com/zscaler-client-connector/what-is-zscaler-client-connector),
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all traffic on the device is automatically routed through Zscaler, so Docker
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Desktop uses the Zscaler proxy automatically with no additional configuration
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necessary.
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If you are not using Zscaler as a system-level proxy, manually configure proxy
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settings in Docker Desktop. Set up proxy settings for all clients in the
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organization using [Settings Management](/manuals/security/for-admins/hardened-desktop/settings-management/_index.md),
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or edit proxy configuration in the Docker Desktop GUI under [**Settings > Resources > Proxies**](/manuals/desktop/settings.md#proxies).
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## Install root certificates in Docker images
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To enable containers to use and trust the Zscaler proxy, embed the certificate
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in the image and configure the image's trust store. Installing certificates at
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image build time is the preferred approach, as it removes the need for
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configuration during startup and provides an auditable, consistent environment.
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### Obtaining the root certificate
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The easiest way to obtain the root certificate is to export it from a machine
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where an administrator has already installed it. You can use either a web
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browser or the system's certificate management service (for example, Windows
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Certificate Store).
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#### Example: Exporting the certificate using Google Chrome
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1. In Google Chrome, navigate to `chrome://certificate-manager/`.
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2. Under **Local certificates**, select **View imported certificates**.
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3. Find the Zscaler root certificate, often labeled **Zscaler Root CA**.
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4. Open the certificate details and select **Export**.
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5. Save the certificate in ASCII PEM format.
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6. Open the exported file in a text editor to confirm it includes `-----BEGIN CERTIFICATE-----` and `-----END CERTIFICATE-----`.
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When you have obtained the certificate, store it in an accessible repository,
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such as JFrog Artifactory or a Git repository. Alternatively, use generic
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storage like AWS S3.
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### Building with the certificate
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To install these certificates when building images, copy the certificate into
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the build container and update the trust store. An example Dockerfile looks
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like this:
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```dockerfile
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FROM debian:bookworm
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COPY zscaler-cert.pem /usr/local/share/ca-certificates/zscaler-cert.pem
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RUN apt-get update && \
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apt-get install -y ca-certificates && \
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update-ca-certificates
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```
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Here, `zscaler-cert.pem` is the root certificate, located at the root of the
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build context (often within the application's Git repository).
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If you use an artifact repository, you can fetch the certificate directly using
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the `ADD` instruction. You can also use the `--checksum` flag to verify that
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the content digest of the certificate is correct.
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```dockerfile
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FROM debian:bookworm
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ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d \
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https://artifacts.example/certs/zscaler-cert.pem /usr/local/share/ca-certificates/zscaler-cert.pem
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RUN apt-get update && \
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apt-get install -y ca-certificates && \
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update-ca-certificates
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```
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#### Using multi-stage builds
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For multi-stage builds where certificates are needed in the final runtime
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image, ensure the certificate installation occurs in the final stage.
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```dockerfile
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FROM debian:bookworm AS build
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WORKDIR /build
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RUN apt-get update && apt-get install -y \
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build-essential \
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cmake \
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curl \
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git
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RUN --mount=target=. cmake -B output/
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FROM debian:bookworm-slim AS final
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ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d \
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https://artifacts.example/certs/zscaler-cert.pem /usr/local/share/ca-certificates/zscaler-cert.pem
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RUN apt-get update && \
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apt-get install -y ca-certificates && \
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update-ca-certificates
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WORKDIR /app
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COPY --from=build /build/output/bin .
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ENTRYPOINT ["/app/bin"]
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```
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## Conclusion
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Embedding the Zscaler root certificate directly into your Docker images ensures
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that containers run smoothly within Zscaler-proxied environments. By using this
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approach, you reduce potential runtime errors and create a consistent,
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auditable configuration that allows for smooth Docker operations within a
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monitored network.

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