Handle docker API errors

app/discovery/provider/docker.go

@@ -324,7 +324,19 @@ func (d *dockerClient) ListContainers() ([]containerInfo, error) {
 
 	defer resp.Body.Close()
 
-	var response []struct {
+	if resp.StatusCode != http.StatusOK {
+		e := struct {
+			Message string `json:"message"`
+		}{}
+
+		if err := json.NewDecoder(resp.Body).Decode(&e); err != nil {
+			return nil, fmt.Errorf("failed to parse error from docker daemon: %w", err)
+		}
+
+		return nil, fmt.Errorf("unexpected error from docker daemon: %s", e.Message)
+	}
+
+	response := []struct {
 		ID              string `json:"Id"`
 		Name            string
 		State           string

app/discovery/provider/docker_test.go

@@ -201,3 +201,16 @@ func TestDockerClient(t *testing.T) {
 
 	assert.Empty(t, c[1].IP)
 }
+
+func TestDockerClient_error(t *testing.T) {
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		http.Error(w, `{"message": "bruh"}`, http.StatusInternalServerError)
+	}))
+
+	defer srv.Close()
+	addr := fmt.Sprintf("tcp://%s", strings.TrimPrefix(srv.URL, "http://"))
+
+	client := NewDockerClient(addr, "bridge")
+	_, err := client.ListContainers()
+	require.EqualError(t, err, "unexpected error from docker daemon: bruh")
+}