makefiles/docker.inc.mk: export RIOT_CI_BUILD to docker

[cladmi]

Contribution description

The variable was not given to docker before and as it is setting
'RIOT_VERSION_OVERRIDE=buildtest' it helps testing in docker more
consistently.

Testing procedure

Compile any example in docker with RIOT_CI_BUILD=1

make BUILD_IN_DOCKER=1 DOCKER="sudo docker" -C examples/hello-world RIOT_CI_BUILD=1

In master you get a verbose output with nothing after the docker make command:

make: Entering directory '/home/harter/work/git/RIOT/examples/hello-world'
Launching build container using image "riot/riotbuild:latest".
sudo docker run --rm -t -u "$(id -u)" \
    -v '/home/harter/work/git/RIOT:/data/riotbuild/riotbase' \
    -v '/home/harter/work/git/RIOT/cpu:/data/riotbuild/riotcpu' \
    -v '/home/harter/work/git/RIOT/boards:/data/riotbuild/riotboard' \
    -v '/home/harter/work/git/RIOT/makefiles:/data/riotbuild/riotmake' \
    -v '/home/harter/work/git/RIOT:/data/riotbuild/riotproject' \
    -v /etc/localtime:/etc/localtime:ro \
    -e 'RIOTBASE=/data/riotbuild/riotbase' \
    -e 'CCACHE_BASEDIR=/data/riotbuild/riotbase' \
    -e 'RIOTCPU=/data/riotbuild/riotcpu' \
    -e 'RIOTBOARD=/data/riotbuild/riotboard' \
    -e 'RIOTMAKE=/data/riotbuild/riotmake' \
    -e 'RIOTPROJECT=/data/riotbuild/riotproject' \
     \
    -w '/data/riotbuild/riotproject/examples/hello-world/' \
    'riot/riotbuild:latest' make  
Building application "hello-world" for "native" with MCU "native".

"make" -C /data/riotbuild/riotbase/core
"make" -C /data/riotbuild/riotbase/drivers
"make" -C /data/riotbuild/riotbase/drivers/periph_common
"make" -C /data/riotbuild/riotbase/sys
"make" -C /data/riotbuild/riotbase/sys/auto_init
"make" -C /data/riotbuild/riotboard/native
"make" -C /data/riotbuild/riotboard/native/drivers
"make" -C /data/riotbuild/riotcpu/native
"make" -C /data/riotbuild/riotcpu/native/periph
"make" -C /data/riotbuild/riotcpu/native/vfs
   text	   data	    bss	    dec	    hex	filename
  20641	    372	  47684	  68697	  10c59	/data/riotbuild/riotproject/examples/hello-world/bin/native/hello-world.elf
make: Leaving directory '/home/harter/work/git/RIOT/examples/hello-world'

And also the version is not overridden by buildtest:

grep 'define RIOT_VERSION' examples/hello-world/bin/native/riotbuild/riotbuild.h
#define RIOT_VERSION "2018.10-devel-734-gf0cf15-4af85705df7a-HEAD"

With this PR you get the more quiet output with make 'RIOT_CI_BUILD=1' docker command:

make: Entering directory '/home/harter/work/git/RIOT/examples/hello-world'
Launching build container using image "riot/riotbuild:latest".
sudo docker run --rm -t -u "$(id -u)" \
    -v '/home/harter/work/git/RIOT:/data/riotbuild/riotbase' \
    -v '/home/harter/work/git/RIOT/cpu:/data/riotbuild/riotcpu' \
    -v '/home/harter/work/git/RIOT/boards:/data/riotbuild/riotboard' \
    -v '/home/harter/work/git/RIOT/makefiles:/data/riotbuild/riotmake' \
    -v '/home/harter/work/git/RIOT:/data/riotbuild/riotproject' \
    -v /etc/localtime:/etc/localtime:ro \
    -e 'RIOTBASE=/data/riotbuild/riotbase' \
    -e 'CCACHE_BASEDIR=/data/riotbuild/riotbase' \
    -e 'RIOTCPU=/data/riotbuild/riotcpu' \
    -e 'RIOTBOARD=/data/riotbuild/riotboard' \
    -e 'RIOTMAKE=/data/riotbuild/riotmake' \
    -e 'RIOTPROJECT=/data/riotbuild/riotproject' \
    -e 'RIOT_CI_BUILD=1' \
    -w '/data/riotbuild/riotproject/examples/hello-world/' \
    'riot/riotbuild:latest' make  'RIOT_CI_BUILD=1'
Building application "hello-world" for "native" with MCU "native".

   text	   data	    bss	    dec	    hex	filename
  20609	    372	  47684	  68665	  10c39	/data/riotbuild/riotproject/examples/hello-world/bin/native/hello-world.elf
make: Leaving directory '/home/harter/work/git/RIOT/examples/hello-world'

And the version is correctly overridden:

grep 'define RIOT_VERSION' examples/hello-world/bin/native/riotbuild/riotbuild.h
#define RIOT_VERSION "buildtest"

Issues/PRs references

Found while working on our docker integration:

#9645
#9646

[kaspar030]

untested ACK.