docs: add building and Makefile usage sections

Signed-off-by: Hank Donnay <hdonnay@redhat.com>

Documentation/SUMMARY.md

@@ -15,8 +15,9 @@
   - [Updaters and Airgap](./concepts/updatersandairgap.md)
   - [Operation](./concepts/operation.md)
 - [Contribution](./contribution.md)
-  - [Releases](./contribution/releases.md)
+  - [Building](./contribution/building.md)
   - [Commit Style](./contribution/commit_style.md)
+  - [Releases](./contribution/releases.md)
 - [Reference](./reference.md)
   - [Api](./reference/api.md)
   - [Clairctl](./reference/clairctl.md)

Documentation/contribution.md

@@ -1,5 +1,6 @@
 # Contribuion
 The following sections provides information on how to contribute to Clair.
 
+- [Building](./contribution/building.md)
 - [Commit Style](./contribution/commit_style.md)
 - [Releases](./contribution/releases.md)

Documentation/contribution/building.md

@@ -0,0 +1,15 @@
+# Building
+
+This repo is intended to be built with familiar `go build` or `go install` invocations.
+All binaries (excepting debugging tools) are underneath the `cmd` directory.
+
+### Cross-compiling
+
+Currently Clair does not have any cgo dependencies, so there should not be any cross-compilation concerns.
+
+## Container
+
+A `Dockerfile` for the project is in the repo root.
+**The only upstream-supported means of using it is Buildkit via `buildctl`.**
+See the `container`, `container-build`, `dist-container`, and `dist-clairctl` make targets for example invocations.
+The `BUILDKIT_HOST` environment variable may need to be set, depending on how `buildkitd` is running in one's environment.

Documentation/contribution/releases.md

@@ -47,3 +47,7 @@ release, and the container is pushed to the
 repository.
 
 This is all powered by a Github Action in `.github/workflows/cut-release.yml`.
+
+A complete source archive can be created with `make dist`.
+A corresponding container can be created with `make dist-container`.
+See `etc/config.mk` for documentation on variables that control these targets.