Dockerfile.alpine: add git and coretuils

git is needed as the top-level Makefile calls it. Strictly speaking
git is not required to build CRIU, but it's better to have it than
to see some error messages.

coreutils is needed for nproc utility which is called from Dockerfile:

	make -j $(nproc)

Before this patch, it was equivalent to "make -j" (plus an error
message) which is not the best thing to do.

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

 FROM alpine:3.4
 RUN apk update && apk add \
                 build-base \
+                coreutils \
+                git \
                 protobuf-c-dev \
                 protobuf-dev \
                 python \