specs-go/config: Make User.UID and User.GID pointers

Avoiding:

  $ go test ./specs-go
  --- FAIL: TestConfigRoundTrip (0.00s)
      --- FAIL: TestConfigRoundTrip/config_2 (0.00s)
          round_trip_test.go:35: failed to round-trip:
                  {"ociVersion":"1.0.0","platform":{"os":"windows","arch":"amd64"},"process":{"user":{"uid":0,"gid":0,"username":"containeradministrator"},"args":["sh"],"cwd":"C:\\"},"root":{"path":"rootfs"}}
  FAIL
  FAIL    github.com/opencontainers/runtime-spec/specs-go 0.003s

We can't just add omitempty (without also making them pointers),
because 0 is a meaningful value for both properties.

Signed-off-by: W. Trevor King <wking@tremily.us>

specs-go/config.go

@@ -82,9 +82,9 @@ type Box struct {
 // User specifies specific user (and group) information for the container process.
 type User struct {
 	// UID is the user id.
-	UID uint32 `json:"uid" platform:"linux,solaris"`
+	UID *uint32 `json:"uid,omitempty" platform:"linux,solaris"`
 	// GID is the group id.
-	GID uint32 `json:"gid" platform:"linux,solaris"`
+	GID *uint32 `json:"gid,omitempty" platform:"linux,solaris"`
 	// AdditionalGids are additional group ids set for the container's process.
 	AdditionalGids []uint32 `json:"additionalGids,omitempty" platform:"linux,solaris"`
 	// Username is the user name.