Docker run -e FOO should erase FOO if FOO isn't set in client env

See #10141 for more info, but the main point of this is to make sure
that if you do "docker run -e FOO ..." that FOO from the current env
is passed into the container.  This means that if there's a value, its
set.  But it also means that if FOO isn't set then it should be unset in
the container too - even if it has to remove it from the env.  So,
   unset HOSTNAME
   docker run -e HOSTNAME busybox env
should _NOT_ show HOSTNAME in the list at all

Closes #10141

Signed-off-by: Doug Davis <dug@us.ibm.com>
Upstream-commit: 67735d2a16
Component: cli

components/cli/opts/opts.go

@@ -11,6 +11,7 @@ import (
 	"github.com/docker/docker/api"
 	flag "github.com/docker/docker/pkg/mflag"
 	"github.com/docker/docker/pkg/parsers"
+	"github.com/docker/docker/utils"
 )
 
 var (
@@ -168,6 +169,9 @@ func ValidateEnv(val string) (string, error) {
 	if len(arr) > 1 {
 		return val, nil
 	}
+	if !utils.DoesEnvExist(val) {
+		return val, nil
+	}
 	return fmt.Sprintf("%s=%s", val, os.Getenv(val)), nil
 }