Update comment to point to culpable commit (#17896)

Marshall Bowers created 

This PR updates the comment added in #17893 to reference the commit that
was identified by `git bisect`.

Release Notes:

- N/A

Change summary

Dockerfile | 2 ++
1 file changed, 2 insertions(+)

Detailed changes

Dockerfile 🔗 

@@ -13,6 +13,8 @@ ENV GITHUB_SHA=$GITHUB_SHA
 # At some point in the past 3 weeks, additional dependencies on `xkbcommon` and
 # `xkbcommon-x11` were introduced into collab.
 #
+# A `git bisect` points to this commit as being the culprit: `b8e6098f60e5dabe98fe8281f993858dacc04a55`.
+#
 # Now when we try to build collab for the Docker image, it fails with the following
 # error:
 #