build: simpler rpm declaration

[conorsch]

The Python app code we ship to dom0 only uses stdlib imports, or else imports that are OS-provided in dom0, suhc as the Qt bindings. Therefore we don't need to use the "setup.py" and "pip install" intermediate build steps, because there's no additional dependency resolution via pip that we care about: we just want to ship *.py files.

This change updates the RPM build logic to use "install" declarations directly, via build-in-place.

Other small improvements:

• tacks on a .dockerignore file as a best practice
• touches up the "hostname" logic in the Makefile, to silence a warning about "hostname" not being on PATH for the container

The built RPM works the same in dom0. I'd actually like to drop the container logic altogether and build directly from system-provided tooling like "rpmbuild", but punting on that now, as it's a more substantive change to build tooling.

Test plan

You should compare an RPM built from this branch with one built from main:

git switch -c review-simple-rpm-before main
make build-rpm
fd --no-ignore -e rpm -x cp {} before.rpm
git switch -c review-simple-rpm-after simpler-rpm-build
make build-rpm
fd --no-ignore -e rpm -x cp {} after.rpm
diffoscope before.rpm after.rpm

and review the output. Additionally, it'd be good to check that a make clean && make clone && make dev works as expected, but that'll take a while.

Checklist

Should be pretty complete. Note that the PR removes mention of the MANIFEST.in file from the description below, because the file itself is removed.

This change accounts for:

• any necessary RPM packaging updates (e.g., added/removed files, see MANIFEST.in and rpm-build/SPECS/securedrop-workstation-dom0-config.spec)
• any required documentation

[conorsch]

These containerfile changes aren't strictly necessary, but they satisfy a few hadolint suggestions, and as long as we're breaking the container image cache by updating the spec file, might as well do some spring cleaning. Refs #1532.