Extract unsupportedOSTypeError to errors pkg

This error is going to be used in multiple places since we only support
a specific set of OS for docker.

helpers/docker/errors/errors.go

+package errors
+
+import (
+	"fmt"
+)
+
+// ErrOSNotSupported is used when docker does not support the detected OSType.
+// NewErrOSNotSupported is used to initialize this type.
+type ErrOSNotSupported struct {
+	detectedOSType string
+}
+
+func (e *ErrOSNotSupported) Error() string {
+	return fmt.Sprintf("unsupported OSType %q", e.detectedOSType)
+}
+
+// NewErrOSNotSupported creates a ErrOSNotSupported for the specified OSType.
+func NewErrOSNotSupported(osType string) *ErrOSNotSupported {
+	return &ErrOSNotSupported{
+		detectedOSType: osType,
+	}
+}

helpers/docker/helperimage/info.go

 package helperimage
 
 import (
-	"fmt"
-
 	"github.com/docker/docker/api/types"
+
+	"gitlab.com/gitlab-org/gitlab-runner/helpers/docker/errors"
 )
 
 const (
@@ -19,20 +19,6 @@ type Info interface {
 	IsSupportingLocalImport() bool
 }
 
-type unsupportedOSTypeError struct {
-	detectedOSType string
-}
-
-func (e *unsupportedOSTypeError) Error() string {
-	return fmt.Sprintf("unsupported OSType %q", e.detectedOSType)
-}
-
-func newUnsupportedOSTypeError(osType string) *unsupportedOSTypeError {
-	return &unsupportedOSTypeError{
-		detectedOSType: osType,
-	}
-}
-
 type infoFactory func(info types.Info) Info
 
 var supportedOsTypesFactories = map[string]infoFactory{
@@ -43,7 +29,7 @@ var supportedOsTypesFactories = map[string]infoFactory{
 func GetInfo(info types.Info) (Info, error) {
 	factory, ok := supportedOsTypesFactories[info.OSType]
 	if !ok {
-		return nil, newUnsupportedOSTypeError(info.OSType)
+		return nil, errors.NewErrOSNotSupported(info.OSType)
 	}
 
 	return factory(info), nil

helpers/docker/helperimage/info_test.go

@@ -5,6 +5,8 @@ import (
 
 	"github.com/docker/docker/api/types"
 	"github.com/stretchr/testify/assert"
+
+	"gitlab.com/gitlab-org/gitlab-runner/helpers/docker/errors"
 )
 
 func TestGetInfo(t *testing.T) {
@@ -15,7 +17,7 @@ func TestGetInfo(t *testing.T) {
 	}{
 		{osType: OSTypeLinux, expectedHelperImageType: &linuxInfo{}, expectedError: nil},
 		{osType: OSTypeWindows, expectedHelperImageType: &windowsInfo{}, expectedError: nil},
-		{osType: "unsupported", expectedHelperImageType: nil, expectedError: newUnsupportedOSTypeError("unsupported")},
+		{osType: "unsupported", expectedHelperImageType: nil, expectedError: errors.NewErrOSNotSupported("unsupported")},
 	}
 
 	for _, testCase := range testCases {