Dockerfile: use 'git clean' before build

The 'make docker-build' command creates a copy of all files from the
in local CRIU clone inside a container.

Then it runs 'make mrproper' inside the container, followed by
compilation of criu, followed by another 'make mrproper'.

After the last mrproper command, it attempts to check if
the clean was successful by running 'git clean'.

However, this check fails when the local repository contains
files that are not part of the repository.

For example, the vscode editor creates the folder '.vscode/'
which would be copied inside the docker container and cause
'make docker-build' to fail.

Signed-off-by: Radostin Stoyanov <rstoyanov@fedoraproject.org>

scripts/build/Dockerfile.tmpl

@@ -29,7 +29,7 @@ RUN apt-install \
 COPY . /criu
 WORKDIR /criu
 
-RUN make mrproper && date && \
+RUN git clean -dfx && date && \
 # Check single object build
 	make -j $(nproc) CC="$CC" criu/parasite-syscall.o && \
 # Compile criu