build deb sources inside a container

A new build target `build-deb` builds mscp deb inside a docker
container. All debuild -us -uc outputs are copied to build/debbuild
directory.

docker/build-deb.Dockerfile

@@ -0,0 +1,20 @@
+FROM ubuntu:22.04
+
+ARG DEBIAN_FRONTEND=noninteractive
+RUN set -ex && apt-get update && apt-get install -y --no-install-recommends \
+	ca-certificates build-essential devscripts debhelper gcc make cmake
+
+ARG mscpdir="/debbuild/mscp"
+
+COPY . ${mscpdir}
+
+# install build dependency
+RUN ${mscpdir}/scripts/install-build-deps.sh
+
+# build
+RUN cd ${mscpdir} 	\
+	&& debuild -us -uc	\
+	&& mv ${mscpdir} /
+
+# Then all debuild output files exsit at /debbuild
+