The ResolveDefaultContext function was exported in [cli@f820766] to allow
(unit) testing, but did not document that it was only exported for this
purpose. The only external use of this function is in buildx, which uses
it in a unit test that can be implemented without this function.
This patch deprecates the function so that we can remove it.
[cli@f820766]: f820766f6a
Signed-off-by: Sebastiaan van Stijn <github@gone.nl>