Add clarifying comment

Author: cschwan

maintainer/Containerfile

@@ -9,6 +9,7 @@ ENV APPL_IGRID_DIR="/usr/local/src/applgrid/src"
 
 # the last version is the default Rust version used in the container
 # as long as we're using `persist-doctests` in the `Rust` workflow we need nightly as default
+# see also https://github.com/rust-lang/rust/issues/56925
 ARG RUST_V="1.80.1 1.91.1 nightly-2026-03-10"
 
 ENV CARGO_HOME="/usr/local/cargo"