Trac #34017: Fix tox-docker builds after #29941

`src/VERSION.txt` is needed in the Docker container after #29941.

This fixes the failures seen in
https://github.com/sagemath/sage/actions/runs/2483481917
and also reported in pypa/setuptools#3389

URL: https://trac.sagemath.org/34017
Reported by: mkoeppe
Ticket author(s): Matthias Koeppe
Reviewer(s): François Bissey

build/bin/write-dockerfile.sh

@@ -213,7 +213,7 @@ WORKDIR sage
 $ADD Makefile VERSION.txt COPYING.txt condarc.yml README.md bootstrap bootstrap-conda configure.ac sage .homebrew-build-env tox.ini Pipfile.m4 ./
 $ADD src/doc/bootstrap src/doc/bootstrap
 $ADD src/bin src/bin
-$ADD src/Pipfile.m4 src/pyproject.toml.m4 src/requirements.txt.m4 src/setup.cfg.m4 src/
+$ADD src/Pipfile.m4 src/pyproject.toml.m4 src/requirements.txt.m4 src/setup.cfg.m4 src/VERSION.txt src/
 $ADD m4 ./m4
 $ADD pkgs pkgs
 $ADD build ./build