docs: add post-install keytool import for the JDK cacerts trust store

Document how to make the installed JDK trust an internal CA at application
runtime by importing it into $JAVA_HOME/lib/security/cacerts with keytool
after setup-java runs. Clarifies this is the runtime trust layer, distinct
from the download/transport layer (NODE_EXTRA_CA_CERTS), and notes hosted vs
self-hosted persistence caveats.

Refs #640 #1035

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

docs/advanced-usage.md

@@ -22,6 +22,7 @@
 - [Hosted Tool Cache](#Hosted-Tool-Cache)
 - [Modifying Maven Toolchains](#Modifying-Maven-Toolchains)
 - [Java-version file](#Java-version-file)
+- [Self-signed certificates and internal CAs (GitHub Enterprise)](#Self-signed-certificates-and-internal-CAs-GitHub-Enterprise)
 
 See [action.yml](../action.yml) for more details on task inputs.
 
@@ -660,3 +661,94 @@ If the file contains multiple versions, only the first one will be recognized.
 
 ***NOTE***:
 For the tool-version file, ensure that you use standard semantic versioning (semver) formats, as non-standard formats (such as jetbrains-21b212.1) may not be parsed correctly. Additionally, for complex version strings containing multiple version-like segments (for example, java semeru-openj9-11.0.15+10_openj9-0.32.0), the extraction logic may incorrectly capture the last segment (0.32.0) instead of the main version (11.0.15+10).
+
+## Self-signed certificates and internal CAs (GitHub Enterprise)
+
+When `setup-java` dynamically downloads a JDK, it makes HTTPS requests both to fetch the available version metadata and to download the JDK archive. If your runners sit behind a **TLS-inspecting corporate proxy**, or you are on **GitHub Enterprise Server (GHES)** with an internal certificate authority, those requests can fail with an error such as:
+
+```
+Error: self signed certificate in certificate chain
+```
+
+This happens because the certificate presented to the runner is signed by an **internal or self-signed CA** that is not part of the runner's default trust store. The download itself is fine — the runner simply cannot verify the certificate chain.
+
+### Recommended fix: trust your internal CA
+
+The secure way to resolve this is to make the runner trust your organization's CA, which keeps TLS verification fully enabled. `setup-java` runs on Node.js, which honors the [`NODE_EXTRA_CA_CERTS`](https://nodejs.org/api/cli.html#node_extra_ca_certsfile) environment variable. Point it at your CA bundle (in PEM format) **before** the `actions/setup-java` step:
+
+```yaml
+steps:
+  # The CA bundle is already present on the runner image in this example.
+  # Alternatively, write it from a secret in a previous step.
+  - name: Trust the internal CA
+    run: echo "NODE_EXTRA_CA_CERTS=/etc/ssl/certs/internal-ca.pem" >> "$GITHUB_ENV"
+
+  - uses: actions/setup-java@v5
+    with:
+      distribution: 'temurin'
+      java-version: '21'
+```
+
+If you keep the certificate in a secret rather than on the runner image, write it to disk first:
+
+```yaml
+steps:
+  - name: Write and trust the internal CA
+    run: |
+      echo "${{ secrets.INTERNAL_CA_PEM }}" > "${RUNNER_TEMP}/internal-ca.pem"
+      echo "NODE_EXTRA_CA_CERTS=${RUNNER_TEMP}/internal-ca.pem" >> "$GITHUB_ENV"
+
+  - uses: actions/setup-java@v5
+    with:
+      distribution: 'temurin'
+      java-version: '21'
+```
+
+For **self-hosted runners**, you can instead install your CA into the operating system's trust store (for example, `update-ca-certificates` on Debian/Ubuntu or `update-ca-trust` on RHEL). This makes the certificate trusted for all tooling on the runner, not just `setup-java`.
+
+### GitHub Enterprise customers
+
+On **GitHub Enterprise Server**, traffic from your runners frequently passes through an organization-managed proxy or terminates TLS at an appliance using a certificate from an internal CA. If your workflows hit the error above, set `NODE_EXTRA_CA_CERTS` to your enterprise CA bundle (or bake the CA into your self-hosted runner image) as shown above. Coordinate with your platform team to obtain the correct PEM bundle for your appliance and proxy chain.
+
+### Security warning: do not disable certificate verification
+
+Do **not** work around this error by disabling TLS verification (for example, by setting `NODE_TLS_REJECT_UNAUTHORIZED=0`). `setup-java` does not verify a pinned checksum or signature of the downloaded archive, so **TLS is effectively the only integrity guarantee** on the JDK download. Disabling verification would expose your workflow to a man-in-the-middle attacker who could serve a tampered JDK — which then becomes the `java` used by the rest of your pipeline, with access to your secrets and credentials. Always extend trust to your CA instead of turning verification off.
+
+### Trusting an internal CA inside the installed JDK
+
+The guidance above makes the **runner** trust your CA so that the JDK can be *downloaded*. That is a separate layer from making the **installed JDK** trust your CA at *application runtime*. If your build steps (Maven/Gradle dependency resolution, integration tests, HTTPS calls from your app, etc.) connect to internal services that present a certificate from your internal CA, the JDK will reject them with errors such as:
+
+```
+PKIX path building failed: unable to find valid certification path to requested target
+```
+
+The JDK keeps its own trust store — a keystore named `cacerts` under `$JAVA_HOME/lib/security/cacerts` — which is independent of the operating system and Node trust stores. After `setup-java` has run (so that `JAVA_HOME` points at the freshly installed JDK), import your CA into that keystore with `keytool`:
+
+```yaml
+steps:
+  - uses: actions/setup-java@v5
+    with:
+      distribution: 'temurin'
+      java-version: '21'
+
+  - name: Import internal CA into the JDK trust store
+    shell: bash
+    run: |
+      # Write the CA from a secret (or reference a file already on the runner)
+      echo "${{ secrets.INTERNAL_CA_PEM }}" > "${RUNNER_TEMP}/internal-ca.pem"
+      keytool -importcert -noprompt \
+        -alias internal-ca \
+        -file "${RUNNER_TEMP}/internal-ca.pem" \
+        -keystore "${JAVA_HOME}/lib/security/cacerts" \
+        -storepass changeit
+```
+
+Notes and caveats:
+
+- The default keystore password for `cacerts` is `changeit` unless your distribution overrides it.
+- On **hosted runners** the change applies only to the current job's JDK and is discarded when the job ends, so include the import step in every job that needs it.
+- On **self-hosted runners**, importing into a tool-cache JDK persists for as long as that cached version remains on the runner; if you want it to survive JDK reinstalls, pre-seed the CA into your runner image or re-run the import step each time.
+- Prefer giving the certificate a stable, descriptive `-alias` so re-runs are idempotent (re-importing the same alias will fail; add `keytool -delete -alias internal-ca ...` first if you re-run within a long-lived runner).
+
+This documents the post-install workflow; there is no dedicated action input for supplying a custom `cacerts` file.
+