Merge pull request kubernetes#62227 from ixdy/document-source-date-epoch

Automatic merge from submit-queue. If you want to cherry-pick this change to another branch, please follow the instructions <a href="https://github.com/kubernetes/community/blob/master/contributors/devel/cherry-picks.md">here</a>.

Add documentation around build reproducibility and SOURCE_DATE_EPOCH

**What this PR does / why we need it**: makes some of our notes about reproducibility a bit more discoverable.

**Release note**:

```release-note
NONE
```

/assign @BenTheElder

build/README.md

@@ -107,4 +107,23 @@ In addition, there are some other tar files that are created:
 
 When building final release tars, they are first staged into `_output/release-stage` before being tar'd up and put into `_output/release-tars`.
 
+## Reproducibility
+`make release`, its variant `make quick-release`, and Bazel all provide a
+hermetic build environment which should provide some level of reproducibility
+for builds. `make` itself is **not** hermetic.
+
+The Kubernetes build environment supports the [`SOURCE_DATE_EPOCH` environment
+variable](https://reproducible-builds.org/specs/source-date-epoch/) specified by
+the Reproducible Builds project, which can be set to a UNIX epoch timestamp.
+This will be used for the build timestamps embedded in compiled Go binaries,
+and maybe someday also Docker images.
+
+One reasonable setting for this variable is to use the commit timestamp from the
+tip of the tree being built; this is what the Kubernetes CI system uses. For
+example, you could use the following one-liner:
+
+```bash
+SOURCE_DATE_EPOCH=$(git show -s --format=format:%ct HEAD)
+```
+
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