Replace gometalinter with GolangCI-Lint

gometalinter is broken @ HEAD, and I looked into why that was. During
that process, I remembered that we took the linting scripts from
skaffold, and found that in skaffold gometalinter was replaced with
GolangCI-Lint:

https://github.com/GoogleContainerTools/skaffold/pull/619

The change made linting in skaffold faster, so I figured instead of
fixing gometalinter it made more sense to remove it and replace it with
GolangCI-Lint for kaniko as well.

pkg/dockerfile/buildargs.go

@@ -17,8 +17,9 @@ limitations under the License.
 package dockerfile
 
 import (
-	d "github.com/docker/docker/builder/dockerfile"
 	"strings"
+
+	d "github.com/docker/docker/builder/dockerfile"
 )
 
 type BuildArgs struct {
@@ -36,14 +37,14 @@ func NewBuildArgs(args []string) *BuildArgs {
 		}
 	}
 	return &BuildArgs{
-		*d.NewBuildArgs(argsFromOptions),
+		BuildArgs: *d.NewBuildArgs(argsFromOptions),
 	}
 }
 
 func (b *BuildArgs) Clone() *BuildArgs {
 	clone := b.BuildArgs.Clone()
 	return &BuildArgs{
-		*clone,
+		BuildArgs: *clone,
 	}
 }