Dockerfile: do make mrproper before build

I just spent some considerable time trying to figure out the source
of errors like this:

	make -C scripts/build ppc64
	...
	  DEP      images/autofs.pb-c.d
	In file included from images/autofs.pb-c.c:9:0:
	images/autofs.pb-c.h:7:35: fatal error: protobuf-c/protobuf-c.h:
	No such	file or directory
	 #include <protobuf-c/protobuf-c.h>
					   ^
	compilation terminated.

It appears that we copy the complete tree to a docker container, and it
is not cleaned properly before the build. In particular, images/*.pb-c.{c,h}
are not removed/rebuilt. The problem is not noticeable until version of
protobuf-c differs between the host and container, and they use different
include paths (/usr/include/google/protobuf-c/ vs /usr/include/protobuf-c).

As I don't want anyone else to spend so much time figuring this out,
here is the patch.

travis-ci: success for QEMU+Dockerfile improvements + clang build
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Acked-by: Andrei Vagin <avagin@virtuozzo.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

scripts/build/Dockerfile.alpine

@@ -12,4 +12,4 @@ RUN apk update && apk add \
                 pkgconfig
 COPY . /criu
 WORKDIR /criu
-RUN make clean && make -j $(nproc)
+RUN make mrproper && make -j $(nproc)

scripts/build/Dockerfile.tmpl

@@ -21,6 +21,6 @@ RUN apt-get install -y \
 COPY . /criu
 WORKDIR /criu
 
-RUN make clean && make -j $(nproc)
+RUN make mrproper && make -j $(nproc)
 RUN make mrproper
 RUN bash -c 'CLEAN="$(git clean -ndx --exclude=scripts/build)"; echo "${CLEAN}"; test -z "${CLEAN}"; exit $?'