Makefile: remove unused DOCKER_BUILD_OPTS

This option was added in e6d7df2e5d to allow
skipping copying the source code as part of `docker build` if a bind-mount
was used to improve the build performance.

This trick was replaced in 1c82d11908, which
now uses a separate target.

With the above, the `DOCKER_BUILD_OPTS` is no longer used, so we can remove it.

Signed-off-by: Sebastiaan van Stijn <github@gone.nl>

Makefile

@@ -28,7 +28,6 @@ DOCKER_ENVS := \
 	-e KEEPBUNDLE \
 	-e DOCKER_BUILD_ARGS \
 	-e DOCKER_BUILD_GOGC \
-	-e DOCKER_BUILD_OPTS \
 	-e DOCKER_BUILD_PKGS \
 	-e DOCKER_BUILDKIT \
 	-e DOCKER_CLI_PATH \
@@ -152,7 +151,7 @@ ifdef FIREWALLD
 DOCKER_BUILD_ARGS += --build-arg=FIREWALLD=true
 endif
 
-BUILD_OPTS := ${DOCKER_BUILD_ARGS} ${DOCKER_BUILD_OPTS}
+BUILD_OPTS := ${DOCKER_BUILD_ARGS}
 BUILD_CMD := $(BUILDX) build
 BAKE_CMD := $(BUILDX) bake