On this ticket, we simplify the code for portability testing using the new options of sage -package list introduced in #30865.

The simplifications to build/bin/write-dockerfile.sh are similar to those done in #30947 for src/doc/bootstrap.

The changes to tox.ini are to redefine the package configurations -minimal, -standard, -maximal using a variable SAGE_PACKAGE_LIST_ARGS - which is passed to sage -package list to obtain a list of packages. (For -minimal, we use SAGE_PACKAGE_LIST_ARGS=_prereq, which produces the empty package list; _prereq is in anticipation of #29124, which creates a script package by this name.)

To test, use for example

    tox -e docker-archlinux-minimal -- config.status
    tox -e docker-ubuntu-bionic-standard -- config.status
    tox -e docker-fedora-32-maximal -- config.status

Depends on #30865
Depends on #30923

CC: @tobiasdiez @seblabbe @kliem

Component: porting

Author: Matthias Koeppe

Branch/Commit: 8a575d4

Reviewer: Sébastien Labbé

Issue created by migration from https://trac.sagemath.org/ticket/30951