refactor(docker): consume the base images instead of building them

The `virgin` stage installed the distribution build dependencies, and that
layer now lives in its own repository:
https://github.com/kevinveenbirkenbach/base-images. What remains here is one
image per distribution that installs pkgmgr on top of it. `slim` goes with it:
it was published for every distribution and pulled by nothing.

scripts/build/base.sh stops pinning five upstream images and resolves
ghcr.io/<owner>/base-<distro>:<tag> instead. Its env names follow the images
repository rather than this one, so the code shows whose namespace that is.
image.sh loses the --target axis, the -virgin/-slim tag suffixes with it, and
passes --platform on push. Manjaro joins the set; os_resolver.sh already maps
it onto arch, so it needs no dependency script of its own.

The two virgin workflows pull the base image rather than building it. What
they prove is unchanged: pkgmgr installs into an untouched container, as root
and as an unprivileged user.

test_distro_dependency_scripts_install_gpg_tools goes: it grepped the
dependency scripts for gnupg, and those scripts are base-images' responsibility
now, where the contract test runs `gpg --version` instead of reading a package
list.

Verified against locally built base images: the arch image builds FROM
base-arch, and the full suite passes inside it - 405 unit, 90 integration and
49 e2e tests.

Requires base-images to be published first; until then every build here fails
at the pull.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

tests/integration/test_git_verification_runtime_dependencies.py

@@ -1,6 +1,5 @@
 from __future__ import annotations
 
-import re
 import unittest
 from pathlib import Path
 
@@ -25,33 +24,6 @@ class TestGitVerificationRuntimeDependencies(unittest.TestCase):
         self.assertIn("pkgs.git", flake_text)
         self.assertIn("pkgs.gnupg", flake_text)
 
-    def test_distro_dependency_scripts_install_gpg_tools(self) -> None:
-        repo_root = _find_repo_root()
-        expected_packages = {
-            "arch": "gnupg",
-            "debian": "gnupg",
-            "ubuntu": "gnupg",
-            "fedora": "gnupg2",
-            "centos": "gnupg2",
-        }
-
-        missing: list[str] = []
-        for distro, package_name in expected_packages.items():
-            script_path = (
-                repo_root / "scripts" / "installation" / distro / "dependencies.sh"
-            )
-            content = script_path.read_text(encoding="utf-8")
-            if not re.search(rf"\b{re.escape(package_name)}\b", content):
-                missing.append(
-                    f"{distro}: expected package {package_name} in {script_path}"
-                )
-
-        if missing:
-            self.fail(
-                "Git signature verification runtime dependencies are incomplete:\n"
-                + "\n".join(f"  - {item}" for item in missing)
-            )
-
 
 if __name__ == "__main__":
     unittest.main()