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/util/bucket_util.go

@@ -17,8 +17,9 @@ limitations under the License.
 package util
 
 import (
-	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 	"strings"
+
+	"github.com/GoogleContainerTools/kaniko/pkg/constants"
 )
 
 func GetBucketAndItem(context string) (string, string) {

pkg/util/command_util_test.go

@@ -23,7 +23,7 @@ import (
 	"github.com/GoogleContainerTools/kaniko/testutil"
 )
 
-var testUrl = "https://github.com/GoogleContainerTools/runtimes-common/blob/master/LICENSE"
+var testURL = "https://github.com/GoogleContainerTools/runtimes-common/blob/master/LICENSE"
 
 var testEnvReplacement = []struct {
 	path         string
@@ -220,7 +220,7 @@ var matchSourcesTests = []struct {
 		srcs: []string{
 			"pkg/*",
 			"/root/dir?",
-			testUrl,
+			testURL,
 		},
 		files: []string{
 			"pkg/a",
@@ -234,7 +234,7 @@ var matchSourcesTests = []struct {
 			"/root/dir1",
 			"pkg/a",
 			"pkg/b",
-			testUrl,
+			testURL,
 		},
 	},
 }
@@ -344,11 +344,11 @@ var isSrcValidTests = []struct {
 	},
 	{
 		srcsAndDest: []string{
-			testUrl,
+			testURL,
 			"dest",
 		},
 		resolvedSources: []string{
-			testUrl,
+			testURL,
 		},
 		shouldErr: false,
 	},
@@ -369,13 +369,13 @@ var testResolveSources = []struct {
 		srcsAndDest: []string{
 			"context/foo",
 			"context/b*",
-			testUrl,
+			testURL,
 			"dest/",
 		},
 		expectedList: []string{
 			"context/foo",
 			"context/bar",
-			testUrl,
+			testURL,
 		},
 	},
 }

pkg/util/fs_util_test.go

@@ -343,8 +343,8 @@ func filesAreHardlinks(first, second string) checker {
 		if err != nil {
 			t.Fatalf("error getting file %s", second)
 		}
-		stat1 := getSyscallStat_t(fi1)
-		stat2 := getSyscallStat_t(fi2)
+		stat1 := getSyscallStatT(fi1)
+		stat2 := getSyscallStatT(fi2)
 		if stat1.Ino != stat2.Ino {
 			t.Errorf("%s and %s aren't hardlinks as they dont' have the same inode", first, second)
 		}

pkg/util/image_util.go

@@ -25,6 +25,7 @@ import (
 	"github.com/google/go-containerregistry/pkg/name"
 	"github.com/google/go-containerregistry/pkg/v1"
 	"github.com/google/go-containerregistry/pkg/v1/empty"
+	"github.com/google/go-containerregistry/pkg/v1/partial"
 	"github.com/google/go-containerregistry/pkg/v1/remote"
 	"github.com/google/go-containerregistry/pkg/v1/tarball"
 	"github.com/sirupsen/logrus"
@@ -61,7 +62,7 @@ func RetrieveSourceImage(stage config.KanikoStage, buildArgs []string) (v1.Image
 }
 
 // RetrieveConfigFile returns the config file for an image
-func RetrieveConfigFile(sourceImage v1.Image) (*v1.ConfigFile, error) {
+func RetrieveConfigFile(sourceImage partial.WithConfigFile) (*v1.ConfigFile, error) {
 	imageConfig, err := sourceImage.ConfigFile()
 	if err != nil {
 		return nil, err

pkg/util/tar_util.go

@@ -118,7 +118,7 @@ func (t *Tar) Whiteout(p string) error {
 func (t *Tar) checkHardlink(p string, i os.FileInfo) (bool, string) {
 	hardlink := false
 	linkDst := ""
-	stat := getSyscallStat_t(i)
+	stat := getSyscallStatT(i)
 	if stat != nil {
 		nlinks := stat.Nlink
 		if nlinks > 1 {
@@ -135,7 +135,7 @@ func (t *Tar) checkHardlink(p string, i os.FileInfo) (bool, string) {
 	return hardlink, linkDst
 }
 
-func getSyscallStat_t(i os.FileInfo) *syscall.Stat_t {
+func getSyscallStatT(i os.FileInfo) *syscall.Stat_t {
 	if sys := i.Sys(); sys != nil {
 		if stat, ok := sys.(*syscall.Stat_t); ok {
 			return stat