fix go-digest to make sure the algorithms are registered

Previously, ioutils imported the crypty/sha256 package, because it was
used by the HashData() utility. As a side-effect of that import, the
sha256 algorithm was registered through its `init()` function.

Now that the HashData() utility is removed, the import is no longer needed
in this package, but some parts of our code depended on the side-effect, and
without this, it fail to recognise the algorithms, unless something else
happens to import crypto/sha256 / crypto/sha512, which made our
tests fail:

```
=== Failed
=== FAIL: reference TestLoad (0.00s)
    store_test.go:53: failed to parse reference: unsupported digest algorithm

=== FAIL: reference TestSave (0.00s)
    store_test.go:82: failed to parse reference: unsupported digest algorithm

=== FAIL: reference TestAddDeleteGet (0.00s)
    store_test.go:174: could not parse reference: unsupported digest algorithm

=== FAIL: reference TestInvalidTags (0.00s)
    store_test.go:355: assertion failed: error is not nil: unsupported digest algorithm
```

While it would be better to do the import in the actual locations where it's
expected, there may be code-paths we overlook, so instead adding the import
here temporarily. Until the PR in go-digest has been merged and released.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

pkg/ioutils/readers.go

@@ -3,6 +3,11 @@ package ioutils // import "github.com/docker/docker/pkg/ioutils"
 import (
 	"context"
 	"io"
+
+	// make sure crypto.SHA256, crypto.sha512 and crypto.SHA384 are registered
+	// TODO remove once https://github.com/opencontainers/go-digest/pull/64 is merged.
+	_ "crypto/sha256"
+	_ "crypto/sha512"
 )
 
 // ReadCloserWrapper wraps an io.Reader, and implements an io.ReadCloser