guides: add docker+zscaler guide

Signed-off-by: David Karlsson <35727626+dvdksn@users.noreply.github.com>

_vale/config/vocabularies/Docker/accept.txt

@@ -99,6 +99,7 @@ OCI
 OTel
 Okta
 PAT
+PEM
 Postgres
 PowerShell
 Python
@@ -140,6 +141,7 @@ WSL
 Wasm
 Windows
 WireMock
+Zscaler
 Zsh
 [Bb]uildx
 [Cc]odenames?

content/guides/zscaler/index.md

@@ -0,0 +1,141 @@
+---
+title: Using Docker with Zscaler
+tags: [networking, admin]
+summary: |
+  This guide explains how to embed Zscaler’s root certificate into Docker
+  images, allowing containers to operate securely with Zscaler proxies and
+  avoid SSL errors.
+params:
+  time: 10 minutes
+---
+
+In many corporate environments, network traffic is intercepted and monitored
+using HTTPS proxies, such as Zscaler. While Zscaler ensures security compliance
+and network control, it can cause issues for developers using Docker,
+particularly during build processes, where SSL certificate validation errors
+might occur. This guide outlines how to configure Docker containers and builds
+to properly handle Zscaler's custom certificates, ensuring smooth operation in
+monitored environments.
+
+## The role of certificates in Docker
+
+When Docker builds or runs containers, it often needs to fetch resources from
+the internet—whether it's pulling a base image from a registry, downloading
+dependencies, or communicating with external services. In a proxied
+environment, Zscaler intercepts HTTPS traffic and replaces the remote server's
+certificate with its own. However, Docker doesn't trust this Zscaler
+certificate by default, leading to SSL errors.
+
+```plaintext
+x509: certificate signed by unknown authority
+```
+
+These errors occur because Docker cannot verify the validity of the certificate
+presented by Zscaler. To avoid this, you must configure Docker to trust
+Zscaler's certificate.
+
+## Configure Zscaler proxy for Docker Desktop
+
+Depending on how Zscaler is deployed, you may need to configure Docker Desktop
+proxy settings manually to use the Zscaler proxy.
+
+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),
+all traffic on the device is automatically routed through Zscaler, so Docker
+Desktop uses the Zscaler proxy automatically with no additional configuration
+necessary.
+
+If you are not using Zscaler as a system-level proxy, manually configure proxy
+settings in Docker Desktop. Set up proxy settings for all clients in the
+organization using [Settings Management](/manuals/security/for-admins/hardened-desktop/settings-management/_index.md),
+or edit proxy configuration in the Docker Desktop GUI under [**Settings > Resources > Proxies**](/manuals/desktop/settings.md#proxies).
+
+## Install root certificates in Docker images
+
+To enable containers to use and trust the Zscaler proxy, embed the certificate
+in the image and configure the image's trust store. Installing certificates at
+image build time is the preferred approach, as it removes the need for
+configuration during startup and provides an auditable, consistent environment.
+
+### Obtaining the root certificate
+
+The easiest way to obtain the root certificate is to export it from a machine
+where an administrator has already installed it. You can use either a web
+browser or the system's certificate management service (for example, Windows
+Certificate Store).
+
+#### Example: Exporting the certificate using Google Chrome
+
+1. In Google Chrome, navigate to `chrome://certificate-manager/`.
+2. Under **Local certificates**, select **View imported certificates**.
+3. Find the Zscaler root certificate, often labeled **Zscaler Root CA**.
+4. Open the certificate details and select **Export**.
+5. Save the certificate in ASCII PEM format.
+6. Open the exported file in a text editor to confirm it includes `-----BEGIN CERTIFICATE-----` and `-----END CERTIFICATE-----`.
+
+When you have obtained the certificate, store it in an accessible repository,
+such as JFrog Artifactory or a Git repository. Alternatively, use generic
+storage like AWS S3.
+
+### Building with the certificate
+
+To install these certificates when building images, copy the certificate into
+the build container and update the trust store. An example Dockerfile looks
+like this:
+
+```dockerfile
+FROM debian:bookworm
+COPY zscaler-cert.pem /usr/local/share/ca-certificates/zscaler-cert.pem
+RUN apt-get update && \
+    apt-get install -y ca-certificates && \
+    update-ca-certificates
+```
+
+Here, `zscaler-cert.pem` is the root certificate, located at the root of the
+build context (often within the application's Git repository).
+
+If you use an artifact repository, you can fetch the certificate directly using
+the `ADD` instruction. You can also use the `--checksum` flag to verify that
+the content digest of the certificate is correct.
+
+```dockerfile
+FROM debian:bookworm
+ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d \
+    https://artifacts.example/certs/zscaler-cert.pem /usr/local/share/ca-certificates/zscaler-cert.pem
+RUN apt-get update && \
+    apt-get install -y ca-certificates && \
+    update-ca-certificates
+```
+
+#### Using multi-stage builds
+
+For multi-stage builds where certificates are needed in the final runtime
+image, ensure the certificate installation occurs in the final stage.
+
+```dockerfile
+FROM debian:bookworm AS build
+WORKDIR /build
+RUN apt-get update && apt-get install -y \
+    build-essential \
+    cmake \
+    curl \
+    git
+RUN --mount=target=. cmake -B output/
+
+FROM debian:bookworm-slim AS final
+ADD --checksum=sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d \
+    https://artifacts.example/certs/zscaler-cert.pem /usr/local/share/ca-certificates/zscaler-cert.pem
+RUN apt-get update && \
+    apt-get install -y ca-certificates && \
+    update-ca-certificates
+WORKDIR /app
+COPY --from=build /build/output/bin .
+ENTRYPOINT ["/app/bin"]
+```
+
+## Conclusion
+
+Embedding the Zscaler root certificate directly into your Docker images ensures
+that containers run smoothly within Zscaler-proxied environments. By using this
+approach, you reduce potential runtime errors and create a consistent,
+auditable configuration that allows for smooth Docker operations within a
+monitored network.