prefer list format for extra_hosts to preserve backward compatibility

Signed-off-by: Nicolas De Loof <nicolas.deloof@gmail.com>

loader/full-struct_test.go

@@ -716,8 +716,8 @@ services:
     - project_db_1:mysql
     - project_db_1:postgresql
     extra_hosts:
-      otherhost: 50.31.209.229
-      somehost: 162.242.195.82
+    - somehost:162.242.195.82
+    - otherhost:50.31.209.229
     hostname: foo
     healthcheck:
       test:
@@ -1302,10 +1302,10 @@ func fullExampleJSON(workingDir, homeDir string) string {
         "project_db_1:mysql",
         "project_db_1:postgresql"
       ],
-      "extra_hosts": {
-        "otherhost": "50.31.209.229",
-        "somehost": "162.242.195.82"
-      },
+      "extra_hosts": [
+        "somehost:162.242.195.82",
+        "otherhost:50.31.209.229"
+      ],
       "hostname": "foo",
       "healthcheck": {
         "test": [

types/types.go

@@ -354,12 +354,12 @@ type ThrottleDevice struct {
 // ShellCommand is a string or list of string args.
 //
 // When marshaled to YAML, nil command fields will be omitted if `omitempty`
-// is specified as a struct tag. Explicitly empty commands (i.e. `[]` or `''`)
+// is specified as a struct tag. Explicitly empty commands (i.e. `[]` or `”`)
 // will serialize to an empty array (`[]`).
 //
 // When marshaled to JSON, the `omitempty` struct must NOT be specified.
 // If the command field is nil, it will be serialized as `null`.
-// Explicitly empty commands (i.e. `[]` or `''`) will serialize to an empty
+// Explicitly empty commands (i.e. `[]` or `”`) will serialize to an empty
 // array (`[]`).
 //
 // The distinction between nil and explicitly empty is important to distinguish
@@ -539,6 +539,14 @@ func (h HostsList) AsList() []string {
 	return l
 }
 
+func (h HostsList) MarshalYAML() (interface{}, error) {
+	return h.AsList(), nil
+}
+
+func (h HostsList) MarshalJSON() ([]byte, error) {
+	return json.Marshal(h.AsList())
+}
+
 // LoggingConfig the logging configuration for a service
 type LoggingConfig struct {
 	Driver  string            `yaml:",omitempty" json:"driver,omitempty"`