Output of `pwd_git_version` produces invalid Docker tag name

[ned2]

The output of pwd_git_version produces a string with a leading @, unlike pwd_version (when using a git version string), which strips off the ampersand.

This means that when version: GIT is set in scrapinghub.yml, when running shub deploy, the image name produced by image/utils.format_image_name() causes the Docker build command to fail with a 500 error: Internal Server Error ("b'invalid reference format'")

A workaround is to use version: AUTO, which uses pwd_version, producing a valid tag name.

Perhaps it would be good to try to standardise the output of pwd_version when git was inferred, and pwd_git_version