fix: preserve ssh:// URL scheme in dockerFilePath

filepath.Join cleans its arguments, collapsing the double slash in
ssh:// URLs to a single slash (ssh:/), corrupting the scheme before
it reaches buildx. Return the dockerfile as-is for any URL-schemed
context, matching the existing behaviour for git:// and https:// URLs.

Fixes the issue #13668

Signed-off-by: Samaresh Kumar Singh <ssam3003@gmail.com>