Makefile.docker: Sort environment variable names

Makefile.docker

@@ -24,38 +24,38 @@ export DOCKER_MAKECMDGOALS ?= all
 # 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 \
+  AR \
+  ARFLAGS \
+  AS \
+  ASFLAGS \
   BINDIR \
+  BINDIRBASE \
+  BOARD \
   BUILDRELPATH \
-  ELFFILE \
-  HEXFILE \
-  LINKFLAGPREFIX \
-  CPPMIX \
-  PREFIX \
+  BUILDTEST_MCU_GROUP \
+  BUILDTEST_VERBOSE \
   CC \
-  CXX \
   CFLAGS \
-  CXXUWFLAGS \
+  CPPMIX \
+  CXX \
   CXXEXFLAGS \
-  AR \
-  ARFLAGS \
-  AS \
-  ASFLAGS \
+  CXXUWFLAGS \
+  ELFFILE \
+  HEXFILE \
   LINK \
+  LINKFLAGPREFIX \
   LINKFLAGS \
   OBJCOPY \
   OFLAGS \
+  PREFIX \
+  QUIET \
+  RIOT_VERSION \
+  SCANBUILD_ARGS \
+  SCANBUILD_OUTPUTDIR \
   SIZE \
-  UNDEF \
-  BUILDTEST_MCU_GROUP \
-  BUILDTEST_VERBOSE \
   TOOLCHAIN \
-  SCANBUILD_OUTPUTDIR \
-  SCANBUILD_ARGS \
+  UNDEF \
   #
 
 # Find which variables were set using the command line or the environment and