docs: Add instructions for GHA container generator (slsa-framework#438)

Signed-off-by: Ian Lewis <ianlewis@google.com>

README.md

@@ -12,30 +12,31 @@
 
 <!-- toc -->
 
--   [Overview](#overview)
-    -   [What is SLSA?](#what-is-slsa)
-    -   [What is provenance?](#what-is-provenance)
-    -   [What is slsa-verifier?](#what-is-slsa-verifier)
--   [Installation](#installation)
-    -   [Compilation from source](#compilation-from-source)
-        -   [Option 1: Install via go](#option-1-install-via-go)
-        -   [Option 2: Compile manually](#option-2-compile-manually)
-    -   [Download the binary](#download-the-binary)
--   [Available options](#available-options)
--   [Option list](#option-list)
-    -   [Option details](#option-details)
--   [Verification for GitHub builders](#verification-for-github-builders)
-    -   [Artifacts](#artifacts)
-    -   [Containers](#containers)
--   [Verification for Google Cloud Build](#verification-for-google-cloud-build)
-    -   [Artifacts](#artifacts-1)
-    -   [Containers](#containers-1)
--   [Known Issues](#known-issues)
-    -   [tuf: invalid key](#tuf-invalid-key)
--   [Technical design](#technical-design)
-    -   [Blog post](#blog-post)
-    -   [Specifications](#specifications)
-    -   [TOCTOU attacks](#toctou-attacks)
+- [Overview](#overview)
+  - [What is SLSA?](#what-is-slsa)
+  - [What is provenance?](#what-is-provenance)
+  - [What is slsa-verifier?](#what-is-slsa-verifier)
+- [Installation](#installation)
+  - [Compilation from source](#compilation-from-source)
+    - [Option 1: Install via go](#option-1-install-via-go)
+    - [Option 2: Compile manually](#option-2-compile-manually)
+  - [Download the binary](#download-the-binary)
+- [Available options](#available-options)
+- [Option list](#option-list)
+  - [Option details](#option-details)
+- [Verification for GitHub builders](#verification-for-github-builders)
+  - [Artifacts](#artifacts)
+  - [Containers](#containers)
+- [Verification for Google Cloud Build](#verification-for-google-cloud-build)
+  - [Artifacts](#artifacts-1)
+  - [Containers](#containers-1)
+- [Known Issues](#known-issues)
+  - [tuf: invalid key](#tuf-invalid-key)
+  - [panic: assignment to entry in nil map](#panic-assignment-to-entry-in-nil-map)
+- [Technical design](#technical-design)
+  - [Blog post](#blog-post)
+  - [Specifications](#specifications)
+  - [TOCTOU attacks](#toctou-attacks)
 
 <!-- tocstop -->
 
@@ -83,31 +84,31 @@ It currently supports verifying provenance generated by:
 
 [Installation](#installation)
 
--   [Compilation from source](#compilation-from-source)
--   [Download the binary](#download-the-binary)
+- [Compilation from source](#compilation-from-source)
+- [Download the binary](#download-the-binary)
 
 [Available options](#available-options)
 
--   [Option list](#option-list)
--   [Option details](#option-details)
+- [Option list](#option-list)
+- [Option details](#option-details)
 
 [Verification for GitHub builders](#verification-for-github-builders)
 
--   [Artifacts](#artifacts)
--   [Containers](#containers)
+- [Artifacts](#artifacts)
+- [Containers](#containers)
 
 [Verification for Google Cloud Build](#verification-for-google-cloud-build)
 
--   [Artifacts](#artifacts-1)
--   [Containers](#containers-1)
+- [Artifacts](#artifacts-1)
+- [Containers](#containers-1)
 
 [Known Issues](#known-issues)
 
 [Technical design](#technial-design)
 
--   [Blog posts](#blog-posts)
--   [Specifications](#specifications)
--   [TOCTOU attacks](#toctou-attacks)
+- [Blog posts](#blog-posts)
+- [Specifications](#specifications)
+- [TOCTOU attacks](#toctou-attacks)
 
 ---
 
@@ -230,7 +231,34 @@ The only requirement is that the provenance file covers all artifacts passed as
 
 ### Containers
 
-This is WIP and currently not supported.
+To verify a container image, you need to pass a container image name that is _immutable_ by providing its digest, in order to avoid [TOCTOU attacks](#toctou-attacks).
+
+First set the image name:
+
+```shell
+IMAGE=ghcr.io/ianlewis/actions-test:v0.0.86
+```
+
+Get the digest for your container _without_ pulling it using the [crane](https://github.com/google/go-containerregistry/blob/main/cmd/crane/doc/crane.md) command:
+
+```shell
+IMAGE="${IMAGE}@"$(crane digest "${IMAGE}")
+```
+
+To verify a container image, run the following command. Note that to use `ghcr.io` you need to set the `GH_TOKEN` environment variable as well.
+
+```shell
+slsa-verifier verify-image "$IMAGE" \
+    --source-uri github.com/ianlewis/actions-test \
+    --source-tag v0.0.86
+```
+
+You should see that the verification passed in the output.
+
+```
+Verified build using builder https://github.com/slsa-framework/slsa-github-generator/.github/workflows/generator_container_slsa3.yml@refs/tags/v1.4.0 at commit d9be953dd17e7f20c7a234ada668f9c8c4aaafc3
+PASSED: Verified SLSA provenance
+```
 
 ## Verification for Google Cloud Build
 
@@ -242,32 +270,36 @@ This is WIP and currently not supported.
 
 To verify a container image, you need to pass a container image name that is _immutable_ by providing its digest, in order to avoid [TOCTOU attacks](#toctou-attacks).
 
-Run the commands below:
+First set the image name:
 
-```bash
-$ IMAGE=laurentsimon/slsa-gcb-v0.3:test
+```shell
+IMAGE=laurentsimon/slsa-gcb-v0.3:test
 ```
 
 Download the provenance:
 
 ```shell
-$ gcloud artifacts docker images describe $IMAGE --format json --show-provenance > provenance.json
+gcloud artifacts docker images describe $IMAGE --format json --show-provenance > provenance.json
 ```
 
 Get the digest for your container _without_ pulling it using the [crane](https://github.com/google/go-containerregistry/blob/main/cmd/crane/doc/crane.md) command:
 
 ```shell
-$ IMAGE="${IMAGE}@"$(crane digest "${IMAGE}")
+IMAGE="${IMAGE}@"$(crane digest "${IMAGE}")
 ```
 
 Verify the image:
 
-```bash
-$ slsa-verifier verify-image "$IMAGE" \
+```shell
+slsa-verifier verify-image "$IMAGE" \
   --provenance-path provenance.json \
   --source-uri github.com/laurentsimon/gcb-tests \
   --builder-id=https://cloudbuild.googleapis.com/GoogleHostedWorker
+```
+
+You should see that the verification passed in the output.
 
+```
 PASSED: Verified SLSA provenance
 ```
 
@@ -291,7 +323,7 @@ FAILED: SLSA verification failed: could not find a matching valid signature entr
 
 This issue is tracked by [issue #325](https://github.com/slsa-framework/slsa-verifier/issues/325). You _must_ update to the newest patch versions of each minor release to fix this issue.
 
-### panic: assignment to entry in nil map 
+### panic: assignment to entry in nil map
 
 This will occur only when verifying provenance against workflow inputs.