docker: fix build with golang 1.16

Add GO111MODULE=auto to GO_PKG_BUILD_VARS to allow the package to be
built in non-module mode.

Module-aware mode will be mandatory in the next golang release.

Signed-off-by: Eneas U de Queiroz <cotequeiroz@gmail.com>

utils/docker/Makefile

@@ -2,7 +2,7 @@ include $(TOPDIR)/rules.mk
 
 PKG_NAME:=docker
 PKG_VERSION:=20.10.2
-PKG_RELEASE:=1
+PKG_RELEASE:=2
 PKG_LICENSE:=Apache-2.0
 PKG_LICENSE_FILES:=LICENSE
 
@@ -34,6 +34,7 @@ define Package/docker/description
 The CLI used in the Docker CE and Docker EE products.
 endef
 
+GO_PKG_BUILD_VARS += GO111MODULE=auto
 TAR_OPTIONS:=--strip-components 1 $(TAR_OPTIONS)
 TAR_CMD=$(HOST_TAR) -C $(1) $(TAR_OPTIONS)