fix: do not rebuild existing containers w/o files

distro/centos/centos.go

@@ -37,10 +37,13 @@ func (centos CentOS) Packages() (pkgs []string, err error) {
 		return
 	}
 
-	err = c.Build("centos:"+centos.release, centos.envs(), centos.runs())
+	if !c.Exist() {
+		err = c.Build("centos:"+centos.release,
+			centos.envs(), centos.runs())
 		if err != nil {
 			return
 		}
+	}
 
 	cmd := "yum search kernel --showduplicates 2>/dev/null " +
 		"| grep '^kernel-[0-9]' " +

distro/debian/debian.go

@@ -54,10 +54,12 @@ func (d Debian) Packages() (packages []string, err error) {
 		return
 	}
 
+	if !c.Exist() {
 		err = c.Build(d.image(), d.envs(), d.runs())
 		if err != nil {
 			return
 		}
+	}
 
 	kernels, err := GetKernels()
 	if err != nil {

distro/opensuse/opensuse.go

@@ -70,10 +70,12 @@ func (suse OpenSUSE) Packages() (pkgs []string, err error) {
 		name = "opensuse/leap:" + suse.release
 	}
 
+	if !c.Exist() {
 		err = c.Build(name, suse.envs(), suse.runs())
 		if err != nil {
 			return
 		}
+	}
 
 	cmd := "zypper search -s --match-exact kernel-default | grep x86_64 " +
 		"| cut -d '|' -f 4 | sed 's/ //g'"

distro/oraclelinux/oraclelinux.go

@@ -38,10 +38,13 @@ func (ol OracleLinux) Packages() (pkgs []string, err error) {
 		return
 	}
 
-	err = c.Build("oraclelinux:"+ol.release, ol.envs(), ol.runs())
+	if !c.Exist() {
+		err = c.Build("oraclelinux:"+ol.release,
+			ol.envs(), ol.runs())
 		if err != nil {
 			return
 		}
+	}
 
 	if ol.release == "8" {
 		// Image for ol9 is required for some kernels

distro/ubuntu/ubuntu.go

@@ -42,10 +42,12 @@ func (u Ubuntu) Packages() (pkgs []string, err error) {
 		return
 	}
 
+	if !c.Exist() {
 		err = c.Build("ubuntu:"+u.release, u.envs(), u.runs())
 		if err != nil {
 			return
 		}
+	}
 
 	cmd := "apt-cache search " +
 		"--names-only '^linux-image-[0-9\\.\\-]*-generic$' " +