build(docker): disable pip_sdist mode in github ci: COPY README.txt dist/* install/dist/ crashes the docker build if the dist directory is missing. This prevents anchore/grype from scanning the image. This is a new issue as it doesn't happen on my system. The README.txt file is there explicitly to prevent COPY from failing if nothing matches the wildcard. This used to work, but .... So I am disabling building from local sdist. This only affects maintainers. Building from sdist is done to generate a docker that matches a source distribution that has yet to be uploaded to pypi. Other use cases can build using the default of local. I haven't found a way to make a dist subdir in the docker build context directory. I can't even figure out how to identify the context directory name inside the build container. Also updated installation docs.

--- a/doc/installation.txt	Sun Sep 24 21:35:47 2023 -0400
+++ b/doc/installation.txt	Sun Sep 24 22:24:51 2023 -0400
@@ -1754,6 +1754,7 @@
   by some Python users.
 
 ``--build-arg="source=pip_sdist"``
+  Disabled.
   This is meant for maintainer/developer use. It installs using
   pip from a source distribution (sdist) tarball built by
   following the RELEASE.txt. It is meant for testing