docker: Refactor Makefile.include, split to Makefile.docker

 - Better support for passing environment variables to the Docker container.
 - Move Docker-related definitions into Makefile.docker.
 - Only environment variables that originates from the command line or
   the environment will be passed to the Docker container.

Makefile.docker

+export DOCKER_IMAGE ?= riot/riotbuild:latest
+export DOCKER_BUILD_ROOT ?= /data/riotbuild
+export DOCKER_FLAGS ?= --rm
+# List of Docker-enabled make goals
+export DOCKER_MAKECMDGOALS_POSSIBLE = \
+  all \
+  #
+export DOCKER_MAKECMDGOALS = $(filter $(MAKECMDGOALS),$(DOCKER_MAKECMDGOALS_POSSIBLE))
+
+# Default target for building inside a Docker container if nothing was given
+export DOCKER_MAKECMDGOALS ?= all
+# List of all exported environment variables that shall be passed on to the
+# Docker container, they will only be passed if they are set from the
+# environment, not if they are only default Makefile values.
+export DOCKER_ENV_VARS = \
+  BINDIRBASE \
+  BOARD \
+  QUIET \
+  RIOT_VERSION \
+  APPDIR \
+  BINDIR \
+  BUILDRELPATH \
+  ELFFILE \
+  HEXFILE \
+  LINKFLAGPREFIX \
+  CPPMIX \
+  PREFIX \
+  CC \
+  CXX \
+  CFLAGS \
+  CXXUWFLAGS \
+  CXXEXFLAGS \
+  AR \
+  ARFLAGS \
+  AS \
+  ASFLAGS \
+  LINK \
+  LINKFLAGS \
+  OBJCOPY \
+  OFLAGS \
+  SIZE \
+  UNDEF \
+  #
+
+# Find which variables were set using the command line or the environment and
+# pass those to Docker.
+# DOCKER_ENVIRONMENT_CMDLINE must be immediately assigned (:=) or otherwise some
+# of the environment variables will be overwritten by Makefile.include and their
+# origin is changed to "file"
+DOCKER_ENVIRONMENT_CMDLINE := $(foreach varname,$(DOCKER_ENV_VARS), \
+    $(if $(filter environment command,$(origin $(varname))), \
+    -e '$(varname)=$($(varname))', \
+    ))
+DOCKER_ENVIRONMENT_CMDLINE := $(strip $(DOCKER_ENVIRONMENT_CMDLINE))
+
+# This will execute `make $(DOCKER_MAKECMDGOALS)` inside a Docker container.
+# We do not push the regular $(MAKECMDGOALS) to the container's make command in
+# order to only perform building inside the container and defer executing any
+# extra commands such as flashing or debugging until after leaving the
+# container.
+# The `flash`, `term`, `debugserver` etc. targets usually require access to
+# hardware which may not be reachable from inside the container.
+..in-docker-container:
+	@$(COLOR_ECHO) '${COLOR_GREEN}Launching build container using image "$(DOCKER_IMAGE)".${COLOR_RESET}'
+	docker run $(DOCKER_FLAGS) -i -t -u "$$(id -u)" \
+	    -v '$(RIOTBASE):$(DOCKER_BUILD_ROOT)/riotbase' \
+	    -v '$(RIOTCPU):$(DOCKER_BUILD_ROOT)/riotcpu' \
+	    -v '$(RIOTBOARD):$(DOCKER_BUILD_ROOT)/riotboard' \
+	    -v '$(RIOTPROJECT):$(DOCKER_BUILD_ROOT)/riotproject' \
+	    -e 'RIOTBASE=$(DOCKER_BUILD_ROOT)/riotbase' \
+	    -e 'RIOTCPU=$(DOCKER_BUILD_ROOT)/riotcpu' \
+	    -e 'RIOTBOARD=$(DOCKER_BUILD_ROOT)/riotboard' \
+	    -e 'RIOTPROJECT=$(DOCKER_BUILD_ROOT)/riotproject' \
+	    $(DOCKER_ENVIRONMENT_CMDLINE) \
+	    -w '$(DOCKER_BUILD_ROOT)/riotproject/$(BUILDRELPATH)' \
+	    '$(DOCKER_IMAGE)' make $(DOCKER_MAKECMDGOALS)

Makefile.include

@@ -13,6 +13,11 @@ RIOTBOARD := $(abspath $(RIOTBOARD))
 RIOTPROJECT ?= $(shell git rev-parse --show-toplevel 2>/dev/null || pwd)
 RIOTPROJECT := $(abspath $(RIOTPROJECT))
 
+# Include Docker settings near the top because we need to build the environment
+# command line before some of the variable origins are overwritten below when
+# using abspath, strip etc.
+include $(RIOTBASE)/Makefile.docker
+
 # Path to the current directory relative to the git root
 BUILDRELPATH ?= $(shell git rev-parse --show-prefix)
 
@@ -159,7 +164,7 @@ BASELIBS += $(USEPKG:%=${BINDIR}%.a)
 ELFFILE ?= $(BINDIR)$(APPLICATION).elf
 HEXFILE ?= $(ELFFILE:.elf=.hex)
 
-# variables used to complie and link c++
+# variables used to compile and link c++
 CPPMIX ?= $(if $(wildcard *.cpp),1,)
 
 # We assume $(LINK) to be gcc-like. Use `LINKFLAGPREFIX :=` for ld-like linker options.
@@ -274,36 +279,6 @@ objdump:
 		exit 1; }
 	$(PREFIX)objdump -S -D -h $(ELFFILE) | less
 
-export DOCKER_IMAGE ?= riot/riotbuild:latest
-export DOCKER_BUILD_ROOT ?= /data/riotbuild
-export DOCKER_FLAGS ?= --rm
-# Default target for building inside a Docker container
-export DOCKER_MAKECMDGOALS ?= all
-# This will execute `make $(DOCKER_MAKECMDGOALS)` inside a Docker container.
-# We do not push the regular $(MAKECMDGOALS) to the container's make command in
-# order to only perform building inside the container and defer executing any
-# extra commands such as flashing or debugging until after leaving the
-# container.
-# The `flash`, `term`, `debugserver` etc. targets usually require access to
-# hardware which may not be reachable from inside the container.
-..in-docker-container:
-	@$(COLOR_ECHO) '${COLOR_GREEN}Launching build container using image "$(DOCKER_IMAGE)".${COLOR_RESET}'
-	docker run $(DOCKER_FLAGS) -i -t -u "$$(id -u)" \
-	    -v '$(RIOTBASE):$(DOCKER_BUILD_ROOT)/riotbase' \
-	    -v '$(RIOTCPU):$(DOCKER_BUILD_ROOT)/riotcpu' \
-	    -v '$(RIOTBOARD):$(DOCKER_BUILD_ROOT)/riotboard' \
-	    -v '$(RIOTPROJECT):$(DOCKER_BUILD_ROOT)/riotproject' \
-	    -e 'RIOTBASE=$(DOCKER_BUILD_ROOT)/riotbase' \
-	    -e 'RIOTCPU=$(DOCKER_BUILD_ROOT)/riotcpu' \
-	    -e 'RIOTBOARD=$(DOCKER_BUILD_ROOT)/riotboard' \
-	    -e 'RIOTPROJECT=$(DOCKER_BUILD_ROOT)/riotproject' \
-	    -e 'BOARD=$(BOARD)' \
-	    -e 'RIOT_VERSION=$(RIOT_VERSION)' \
-	    -e '__RIOTBUILD_FLAG=$(__RIOTBUILD_FLAG)' \
-	    -e 'QUIET=$(QUIET)' \
-	    -w '$(DOCKER_BUILD_ROOT)/riotproject/$(BUILDRELPATH)' \
-	    '$(DOCKER_IMAGE)' make $(DOCKER_MAKECMDGOALS)
-
 # Extra make goals for testing and comparing changes.
 include $(RIOTBASE)/Makefile.buildtests