Build umbrella Phoenix apps' assets in the Elixir stack

The Elixir stack (MIR-1404) builds Phoenix assets by running mix assets.deploy at the project root when the root mix.exs defines that alias, and installs npm deps from a root assets/package.json. In a mix phx.new --umbrella project, both live under apps/<name>_web/, so neither happens. The stack does detect Phoenix from the root mix.lock, so the release still gets PHX_SERVER and a generated SECRET_KEY_BASE. Assets are the only gap, and the guide currently points at [build] onbuild = ["cd apps/my_app_web && mix assets.deploy"] as the workaround.

Worth doing properly because long-lived Elixir apps are often umbrellas. The stack would find each app under apps/ that defines assets.deploy (and any apps/*/assets/package.json), install its npm deps, and run the alias in that app's directory before mix release. It also needs a Docker test on an umbrella fixture, since none of the current build tests cover one.