cli/compose/schema: make version: "3" equivalent to "3.x" (latest)

Previously, `version: "3"` was equivalent to `version: "3.0"`, which
caused confusion for many users, as they expected it to be "3.x".

docker-compose and docker compose (v2) have adopted the compose-spec
(https://compose-spec.io), which no longer has a version field in
the compose file, and always picks the "latest" supported version.

This changes how `docker stack` interprets "major" version numbers
specified in compose-files:

When only the major version ("3") is specified, it is now equivalent
to "3.x" (latest supported v3 schema).

Compose-files that specify both major and minor version (e.g. "3.0"
or "3.1") continue to use the existing behavior; validation is down-
graded to the specified version and will produce an error if options
are used that are not supported in that schema version. This allows
users to locally verify that a composse-file does not use options
that are not supported in the intended deployment environment (for
example if the deploy environment only supports older versions of
the schema).

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

cli/compose/loader/loader_test.go

@@ -180,7 +180,7 @@ func strPtr(val string) *string {
 }
 
 var sampleConfig = types.Config{
-	Version: "3.0",
+	Version: "3.10",
 	Services: []types.ServiceConfig{
 		{
 			Name:        "foo",

cli/compose/schema/schema.go

@@ -40,7 +40,8 @@ func init() {
 }
 
 // Version returns the version of the config, defaulting to the latest "3.x"
-// version (3.10).
+// version (3.10). If only the major version "3" is specified, it is used as
+// version "3.x" and returns the default version (latest 3.x).
 func Version(config map[string]interface{}) string {
 	version, ok := config[versionField]
 	if !ok {
@@ -51,10 +52,8 @@ func Version(config map[string]interface{}) string {
 
 func normalizeVersion(version string) string {
 	switch version {
-	case "":
+	case "", "3":
 		return defaultVersion
-	case "3":
-		return "3.0"
 	default:
 		return version
 	}

cli/compose/schema/schema_test.go

@@ -88,7 +88,6 @@ func TestValidateCredentialSpecs(t *testing.T) {
 		version     string
 		expectedErr string
 	}{
-		{version: "3", expectedErr: "credential_spec"},
 		{version: "3.0", expectedErr: "credential_spec"},
 		{version: "3.1", expectedErr: "credential_spec"},
 		{version: "3.2", expectedErr: "credential_spec"},
@@ -100,6 +99,7 @@ func TestValidateCredentialSpecs(t *testing.T) {
 		{version: "3.8"},
 		{version: "3.9"},
 		{version: "3.10"},
+		{version: "3"},
 		{version: ""},
 	}