Make screen default to bash explicitly

Implictly I've never needed this as my user shell has always been bash
but in some containers recently the default shell is /bin/sh

screen/.screenrc

@@ -1,3 +1,5 @@
+# Defaults to using bash as the shell
+defshell bash
 # Extend scrollback buffer
 defscrollback 100000
 # Disable startup message