Add user agent header to all requests

Currently the user agent is only being used on the initial
resolve request, then switching to the default user agent.
This ensures the correct user agent is always used. There is
a larger fix in progress which does this is a cleaner way, but
the scope of this change is fixing the user agent issue.

Signed-off-by: Derek McGowan <derek@mcgstyle.net>

remotes/docker/authorizer.go

@@ -31,6 +31,7 @@ import (
 
 	"github.com/containerd/containerd/errdefs"
 	"github.com/containerd/containerd/log"
+	"github.com/containerd/containerd/version"
 	"github.com/pkg/errors"
 	"github.com/sirupsen/logrus"
 	"golang.org/x/net/context/ctxhttp"
@@ -40,6 +41,7 @@ type dockerAuthorizer struct {
 	credentials func(string) (string, string, error)
 
 	client *http.Client
+	ua     string
 	mu     sync.Mutex
 
 	auth map[string]string
@@ -54,6 +56,7 @@ func NewAuthorizer(client *http.Client, f func(string) (string, string, error))
 	return &dockerAuthorizer{
 		credentials: f,
 		client:      client,
+		ua:          "containerd/" + version.Version,
 		auth:        map[string]string{},
 	}
 }
@@ -194,11 +197,16 @@ func (a *dockerAuthorizer) fetchTokenWithOAuth(ctx context.Context, to tokenOpti
 		form.Set("password", to.secret)
 	}
 
-	resp, err := ctxhttp.Post(
-		ctx, a.client, to.realm,
-		"application/x-www-form-urlencoded; charset=utf-8",
-		strings.NewReader(form.Encode()),
-	)
+	req, err := http.NewRequest("POST", to.realm, strings.NewReader(form.Encode()))
+	if err != nil {
+		return "", err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded; charset=utf-8")
+	if a.ua != "" {
+		req.Header.Set("User-Agent", a.ua)
+	}
+
+	resp, err := ctxhttp.Do(ctx, a.client, req)
 	if err != nil {
 		return "", err
 	}
@@ -244,6 +252,10 @@ func (a *dockerAuthorizer) fetchToken(ctx context.Context, to tokenOptions) (str
 		return "", err
 	}
 
+	if a.ua != "" {
+		req.Header.Set("User-Agent", a.ua)
+	}
+
 	reqParams := req.URL.Query()
 
 	if to.service != "" {