Add documentation to authenticate to private go modules

README.md

@@ -1,7 +1,7 @@
 # Go Dependency Submission
 
 This GitHub Action calculates dependencies for a Go build-target (a Go file with a
-`main` function) and submits the list to the [Dependency submission API](https://docs.github.com/en/code-security/supply-chain-security/understanding-your-software-supply-chain/using-the-dependency-submission-api). Dependencies then appear in your repository's dependency graph, and you'll receive Dependabot alerts and updates for vulnerable or out-of-date dependencies. 
+`main` function) and submits the list to the [Dependency submission API](https://docs.github.com/en/code-security/supply-chain-security/understanding-your-software-supply-chain/using-the-dependency-submission-api). Dependencies then appear in your repository's dependency graph, and you'll receive Dependabot alerts and updates for vulnerable or out-of-date dependencies.
 
 ### Running locally
 
@@ -53,3 +53,81 @@ jobs:
             # include Go dependencies used by tests and tooling.
             go-build-target: go-example/cmd/octocat.go
 ```
+
+### Accessing Private Go Modules
+
+This action will fail if your `go.mod` contains private Go modules.
+To access private Go modules the action requires an extra step to access your
+private module registry.
+
+See official Go documentation for more information at https://go.dev/doc/faq#git_https
+
+#### Accessing Private Go Modules Hosted on GitHub
+
+If your private Go modules are hosted on GitHub, there are various ways for the action to
+access them. You can use either HTTPS authentication with a Personal Access Token (PAT) or SSH authentication with deploy keys or SSH keys.
+
+#### Authentication Methods
+
+**HTTPS with Personal Access Token**: Uses a GitHub Personal Access Token to authenticate with private repositories. This method requires storing the token as a repository secret. See [Creating a personal access token](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens#creating-a-personal-access-token-classic) for setup instructions.
+
+**SSH Authentication**: Uses SSH keys or deploy keys for authentication. This method doesn't require storing tokens and can be more secure for some use cases. See the [GitHub documentation on SSH authentication](https://docs.github.com/en/authentication/connecting-to-github-with-ssh) for setup instructions.
+
+#### Additional Environment Variables
+
+- **`GONOPROXY`**: Set this to bypass the module proxy entirely for specific modules
+- **`GOSUMDB`**: Set to `off` or configure to skip checksum verification for private modules
+- **`GOPROXY`**: Can be set to `direct` to bypass proxies completely
+
+#### Example: HTTPS Authentication with Personal Access Token
+
+This example adds a step to the workflow which uses a GitHub
+[Personal Access Token (PAT)](https://docs.github.com/en/authentication/keeping-your-account-and-data-secure/managing-your-personal-access-tokens)
+which has repo permissions. The PAT is saved as a repo [actions secret](https://docs.github.com/en/actions/security-for-github-actions/security-guides/using-secrets-in-github-actions) `GH_ACCESS_TOKEN`.
+
+The env variable `GOPRIVATE` has also been set so that the GitHub org `foo` is considered private.
+
+```yaml
+name: Go Dependency Submission
+on:
+  push:
+    branches:
+      - main
+
+# The API requires write permission on the repository to submit dependencies
+permissions:
+  contents: write
+
+# Environment variables to configure Go and Go modules. Customize as necessary
+env:
+  GOPROXY: 'https://proxy.golang.org,direct' # To add a private proxy, place it between the public golang proxy and direct
+  GOPRIVATE: 'github.com/foo/*' # repositories in organization foo are considered private
+
+jobs:
+  go-action-detection:
+    runs-on: ubuntu-latest
+    steps:
+      - name: 'Checkout Repository'
+        uses: actions/checkout@v3
+
+      - uses: actions/setup-go@v3
+        with:
+          go-version: ">=1.18.0"
+
+      # Authentication step
+      - name: Authenticate with GitHub
+        run: git config --global url.https://${{ secrets.GH_ACCESS_TOKEN }}@github.com/.insteadOf https://github.com/
+
+      - name: Run snapshot action
+        uses: actions/go-dependency-submission@v2
+        with:
+            # Required: Define the repo path to the go.mod file used by the
+            # build target
+            go-mod-path: go-example/go.mod
+            #
+            # Optional: Define the path of a build target (a file with a
+            # `main()` function) If not defined, this Action will collect all
+            # dependencies used by all build targets for the module, which may
+            # include Go dependencies used by tests and tooling.
+            go-build-target: go-example/cmd/octocat.go
+```