service/ci.md

@@ -313,9 +313,16 @@ configuration didn't request an `i386` and would be instead expected to run
 with an `amd64` image. This issue is tracked in tpo/tpa/team#41621.
 
 The workaround is to configure jobs to pull an architecture-specific version of
-their image (eg. `amd64/debian:stable`) instead of the multi-arch manifest (eg.
-`debian:stable`). Note that not all organizations provide such arch-specific
-versions.
+the image instead of one using a multi-arch manifest. For Docker Official
+Images, this can be done by prefixing with `amd64/`; e.g. `amd64/debian:stable`
+instead of `debian:stable`. See GitHub's ["Architectures other than
+amd64"](https://github.com/docker-library/official-images#architectures-other-than-amd64).
+
+When trying to check what arch the current container is built for, `uname -m`
+*doesn't* work, since that gives the arch of the host kernel, which can still
+be `amd64` inside of an `i386` container. You can instead use `dpkg
+--print-architecture` (for debian-based images), or `apk --print-arch` (for
+alpine-based images).
 
 ## Disaster recovery