Clarify requirements for identifiers validation

Fix commentary to be clear that the RFC1035 2.3.1 rules are not
explicitly enforced by the identifiers regex.

Add a test to show the lessened restrictions for digits to follow hyphen
and period.

Signed-off-by: Phil Estes <estesp@linux.vnet.ibm.com>

identifiers/validate_test.go

@@ -34,6 +34,7 @@ func TestValidIdentifiers(t *testing.T) {
 		"swarmkit.docker.io",
 		"0912341234",
 		"task.0.0123456789",
+		"container.system-75-f19a.00",
 		"underscores_are_allowed",
 		strings.Repeat("a", maxLength),
 	} {