Thanks Chris,
I think the diff shows the opposite direction; adding bullseye-20220801-slim, the image which did not exist for my elixir/erlang combination. phoenix 1.6.11..1.6.12 | hexdiff
(1.6 branch on Github shows the same use of the not-found 2022 version.)
Phoenix Master on Github uses the bullseye-20210902-slim debian version that works for me and you indicated is the desired outcome.