cmd/mbf: migrate shell to enter

This reduces duplicate code. This change also adds resolv.conf to the container.

Signed-off-by: Ophestra <cat@gensokyo.uk>

internal/pkg/exec.go

@@ -591,6 +591,7 @@ var (
 func (c *Cache) EnterExec(
 	ctx context.Context,
 	a Artifact,
+	hostname string,
 	retainSession bool,
 	stdin io.Reader,
 	stdout, stderr io.Writer,
@@ -671,6 +672,9 @@ func (c *Cache) EnterExec(
 			z.Env = append(z.Env, "TERM="+s)
 		}
 	}
+	if hostname != "" {
+		z.Hostname = hostname
+	}
 
 	if err = z.Start(); err != nil {
 		return