Only set DOCKER_CROSSPLATFORMS from env if it's set
This allows for the default to come from the image/`Dockerfile` rather
than being unconditionally overwritten by the environment.
@@ -17,13 +17,13 @@ export DOCKER_GITCOMMIT
# to allow things like `make KEEPBUNDLE=1 binary` easily
# `project/PACKAGERS.md` have some limited documentation of some of these
DOCKER_ENVS := \
+ $(if $(DOCKER_CROSSPLATFORMS), -e DOCKER_CROSSPLATFORMS) \
-e BUILD_APT_MIRROR \
-e BUILDFLAGS \
-e KEEPBUNDLE \
-e DOCKER_BUILD_ARGS \
-e DOCKER_BUILD_GOGC \
-e DOCKER_BUILD_PKGS \
- -e DOCKER_CROSSPLATFORMS \
-e DOCKER_DEBUG \
-e DOCKER_EXPERIMENTAL \
-e DOCKER_GITCOMMIT \
