proxy needs to add weavewait to execs as well

Bug came up in a test where we are restarting and doing an exec checking
a network interface right away. weavewait is only added if the container
has a WEAVE_CIDR var set, the same as at container creation.

proxy/create_exec_interceptor.go

@@ -0,0 +1,51 @@
+package proxy
+
+import (
+	"encoding/json"
+	"io/ioutil"
+	"net/http"
+
+	"github.com/fsouza/go-dockerclient"
+)
+
+type createExecInterceptor struct {
+	client *docker.Client
+}
+
+type createExecRequestBody struct {
+	*docker.Config
+	HostConfig *docker.HostConfig `json:"HostConfig,omitempty" yaml:"HostConfig,omitempty"`
+	MacAddress string             `json:"MacAddress,omitempty" yaml:"MacAddress,omitempty"`
+}
+
+func (i *createExecInterceptor) InterceptRequest(r *http.Request) error {
+	body, err := ioutil.ReadAll(r.Body)
+	if err != nil {
+		return err
+	}
+	r.Body.Close()
+
+	options := docker.CreateExecOptions{}
+	if err := json.Unmarshal(body, &options); err != nil {
+		return err
+	}
+
+	container, err := inspectContainerInPath(i.client, r.URL.Path)
+	if err != nil {
+		return err
+	}
+
+	if _, ok := weaveCIDRsFromConfig(container.Config); ok {
+		options.Cmd = append(weaveWaitEntrypoint, options.Cmd...)
+	}
+
+	if err := marshalRequestBody(r, options); err != nil {
+		return err
+	}
+
+	return nil
+}
+
+func (i *createExecInterceptor) InterceptResponse(r *http.Response) error {
+	return nil
+}