Builders: allow COPY with skaffold when the source directory is empty

Required for NXP-27514 "Build Nuxeo Content platform Docker image" to allow copying a potentially empty docker/packages directory.

Updating skaffold to v0.38.0 should fix this, see https://github.com/GoogleContainerTools/skaffold/issues/741.

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In fact, it's a misunderstanding.
Running COPY with an existing but empty source directory works fine, with skaffold or directly using docker.
Yet, a problem occurs if the source directory doesn't exist, again, with skaffold as well as docker, but this is expected.
So, in our case, a non existing directory will fail the build which is what we want since it should exist. An empty directory will just lead to no package installation.

Anyway, let's update skaffold to the latest released version v0.38.0, now that the following error introduced in v0.36.0 was fixed, see https://github.com/GoogleContainerTools/skaffold/issues/2732:

level=fatal msg="build failed: build failed: setting up pull secret: getting kubernetes client: getting client config for kubernetes client: error creating REST client config: invalid configuration: no configuration has been provided