[mod] ./manage docker.buildx make variables local

The environment variables ..

- SEARX_GIT_VERSION
- VERSION_GITCOMMIT
- SEARX_PYTHON_VERSION
- GITHUB_USER
- SEARX_IMAGE_NAME
- BUILD

are local to the docker.buildx() function, other variables like GIT_URL are
comming from the utils/brand.env file (rebuild by: make buildenv).

Signed-off-by: Markus Heiser <markus.heiser@darmarit.de>