add install-build-deps.sh and fix docker build procedure

docker/Dockerfile-centos-8

@@ -1,7 +1,8 @@
 FROM centos:8
 
-ARG DEBIAN_FRONTEND=noninteractive
-ARG workdir="/"
+ARG mscpdir="/mscp"
+
+COPY . ${mscpdir}
 
 # from https://stackoverflow.com/questions/70963985/error-failed-to-download-metadata-for-repo-appstream-cannot-prepare-internal
 RUN cd /etc/yum.repos.d/
@@ -9,10 +10,15 @@ RUN sed -i 's/mirrorlist/#mirrorlist/g' /etc/yum.repos.d/CentOS-*
 RUN sed -i 's|#baseurl=http://mirror.centos.org|baseurl=http://vault.centos.org|g' /etc/yum.repos.d/CentOS-*
 
 RUN set -ex && yum -y update && yum -y install \
-	git cmake gcc make libssh-devel rpm-build
+	rpm-build
+
+RUN ${mscpdir}/install-build-deps.sh
+
+RUN cd ${mscpdir}       \
+        && rm -rf build
+
+RUN cd ${mscpdir}       \
+        && cmake -B build -DBUILD_PKG=1 \
+        && cd ${mscpdir}/build          \
+	&&cpack -G RPM CPackConfig.cmake
 
-RUN cd ${workdir}	\
-	&& git clone --depth=1 https://github.com/upa/mscp	\
-	&& mkdir mscp/build && cd mscp/build	\
-	&& cmake .. -DBUILD_PKG=1 \
-	&& cpack -G RPM CPackConfig.cmake