add ubuntu 16.04 support

Signed-off-by: Adphi <philippe.adrien.nousse@gmail.com>

dockerfile.go

@@ -91,7 +91,11 @@ func NewDockerfile(release OSRelease, img, password string, networkManager Netwo
 		net = NetworkManagerIfupdown2
 	case ReleaseUbuntu:
 		d.tmpl = ubuntuDockerfileTemplate
-		net = NetworkManagerNetplan
+		if release.VersionID < "18.04" {
+			net = NetworkManagerIfupdown2
+		} else {
+			net = NetworkManagerNetplan
+		}
 	case ReleaseAlpine:
 		d.tmpl = alpineDockerfileTemplate
 		net = NetworkManagerIfupdown2

templates/ubuntu.Dockerfile

@@ -25,7 +25,9 @@ RUN ARCH="$([ "$(uname -m)" = "x86_64" ] && echo amd64 || echo arm64)"; \
   iputils-ping && \
   find /boot -type l -exec rm {} \;
 
+{{ if gt .Release.VersionID "16.04" }}
 RUN systemctl preset-all
+{{ end }}
 
 {{ if .Password }}RUN echo "root:{{ .Password }}" | chpasswd {{ end }}