Add .golangci.yml linter configuration

As part of https://gitlab.com/gitlab-org/gitlab-pages/-/issues/385
we have introduced the use of a custom `.golangci.yml` file with some
custom rules for linting.

This replaces the need of downloading and using `golint`, `gofmt`
`go vet` and `gocyclo` manually. We take advantage of the custom
`golangci-lint` docker image as stated in the [Automatic lintinb]
(https://docs.gitlab.com/ee/development/go_guide/#automatic-linting)
section of the Go standards and style guidelines.

This iteration enables a subset of linters, with the remaining
of them enabled on a separate MR as described in the issue above.

The main changes introduced by this linter include:

- gosec: potential hardcoded credentials
- goconst: DRY by declaring and using constants
- gosimple: reduce statements complexity and improve return statements

1 files changed, 1 insertions, 4 deletions

diff --git a/multi_string_flag_test.go b/multi_string_flag_test.go
index 5cddac98..c09f7225 100644
--- a/multi_string_flag_test.go
+++ b/multi_string_flag_test.go
@@ -1,7 +1,6 @@
 package main
 
 import (
-	"flag"
 	"testing"
 
 	"github.com/stretchr/testify/require"
@@ -9,9 +8,7 @@ import (
 
 func TestMultiStringFlagAppendsOnSet(t *testing.T) {
 	var concrete MultiStringFlag
-	var iface flag.Value
-
-	iface = &concrete
+	iface := &concrete
 
 	require.NoError(t, iface.Set("foo"))
 	require.NoError(t, iface.Set("bar"))