rootfs: Add support to create dockerfiles based on templates

To simplify maintaince, create dockerfiles based on templates.

This way when golang version is updated it will be done in one place
versions.txt.

This also allow to allways intall the same version of golang in any
dockerfile.

Signed-off-by: Jose Carlos Venegas Munoz <jose.carlos.venegas.munoz@intel.com>

rootfs-builder/rootfs.sh

@@ -12,6 +12,9 @@ ROOTFS_DIR=${ROOTFS_DIR:-${PWD}/rootfs}
 AGENT_VERSION=${AGENT_VERSION:-master}
 GO_AGENT_PKG=${GO_AGENT_PKG:-github.com/kata-containers/agent}
 AGENT_BIN=${AGENT_BIN:-kata-agent}
+#Load default vesions for golang and other componets
+source "${script_dir}/versions.txt"
+
 # Name of file that will implement build_rootfs
 typeset -r LIB_SH="rootfs_lib.sh"
 
@@ -79,6 +82,29 @@ check_function_exist() {
 	[ "$(type -t ${function_name})" == "function" ] || die "${function_name} function was not defined"
 }
 
+generate_dockerfile() {
+	dir="$1"
+
+	readonly install_go="
+ADD https://storage.googleapis.com/golang/go${GO_VERSION}.linux-amd64.tar.gz /tmp
+RUN tar -C /usr/ -xzf /tmp/go${GO_VERSION}.linux-amd64.tar.gz
+ENV GOROOT=/usr/go
+ENV PATH=\$PATH:\$GOROOT/bin:\$GOPATH/bin
+"
+
+	readonly dockerfile_template="Dockerfile.in"
+	[ -d "${dir}" ] || die "${dir}: not a directory"
+	pushd ${dir}
+	[ -f "${dockerfile_template}" ] || die "${dockerfile_template}: file not found"
+	set -x
+	sed \
+		-e "s|@OS_VERSION@|${OS_VERSION}|g" \
+		-e "s|@INSTALL_GO@|${install_go//$'\n'/\\n}|g" \
+		${dockerfile_template} > Dockerfile
+	set +x
+	popd
+}
+
 
 while getopts c:hr: opt
 do
@@ -110,6 +136,7 @@ check_function_exist "build_rootfs"
 if [ -n "${USE_DOCKER}" ] ; then
 	image_name="${distro}-rootfs-osbuilder"
 
+	generate_dockerfile "${distro_config_dir}"
 	docker build  \
 		--build-arg http_proxy="${http_proxy}" \
 		--build-arg https_proxy="${https_proxy}" \