docker auth: use GetAllCredentials() to use credHelpers
docker auth: use GetAllCredentials() to use credHelpers

The current mechanism for building the map of `AuthConfig` to pass to
the docker daemon when building the image bypasses any credsHelpers in
the user's docker config json file.

`cf.GetCredentialsStore("").GetAll()` ends up returning the `auths`
section from the config file, and does not use or execute any configured
`credHelpers`.

There is a comment in the source code here for a commented-out call to
GetAllCredentials() that it is too slow, but I think it is worth noting
that this is the same function used by a regular `docker build`:
https://github.com/docker/cli/blob/e06530297ddd4d0a4baf4361fc094f4afb36de92/cli/command/image/build.go#L403

While it might be slow to execute each of the configured credHelpers to
be able to build a full map of `AuthConfigs`, this seems like the only
way to pass all of the user's auth configs from their config file to the
build command - otherwise any image pulls needed to build the image will
fail if they require authentication.

Fixes #239.

GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold,GoogleContainerTools/skaffold