Update all Ubuntu definition files

Updated all derivative Ubuntu definition files to bootstrap from the
base Ubuntu 18.04 container that is part of the new naked-singularity
container collection on Singularity Hub.

definition-files/ubuntu/Singularity.ubuntu-18.04-cuda-10.1.168

 Bootstrap: shub
-From: mkandes/ubuntu:ubuntu-18.04
+From: mkandes/naked-singularity:ubuntu-18.04
 
 %labels
 
@@ -10,7 +10,7 @@ From: mkandes/ubuntu:ubuntu-18.04
     AUTHOR_NAME Marty Kandes
     AUTHOR_EMAIL mkandes@sdsc.edu
 
-    LAST_UPDATED 20200714
+    LAST_UPDATED 20201226
 
 %setup
 

definition-files/ubuntu/Singularity.ubuntu-18.04-mvapich-2.3.2

 Bootstrap: shub
-From: mkandes/ubuntu:ubuntu-18.04
+From: mkandes/naked-singularity:ubuntu-18.04
 
 %labels
 
@@ -10,7 +10,7 @@ From: mkandes/ubuntu:ubuntu-18.04
     AUTHOR_NAME Marty Kandes
     AUTHOR_EMAIL mkandes@sdsc.edu
 
-    LAST_UPDATED 20200714
+    LAST_UPDATED 20201226
 
 %setup
 

definition-files/ubuntu/Singularity.ubuntu-18.04-openmpi-3.1.4

 Bootstrap: shub
-From: mkandes/ubuntu:ubuntu-18.04
+From: mkandes/naked-singularity:ubuntu-18.04
 
 %labels
 
@@ -10,7 +10,7 @@ From: mkandes/ubuntu:ubuntu-18.04
     AUTHOR_NAME Marty Kandes
     AUTHOR_EMAIL mkandes@sdsc.edu
 
-    LAST_UPDATED 20200809
+    LAST_UPDATED 20201226
 
 %setup
 

definition-files/ubuntu/Singularity.ubuntu-18.04-openmpi-3.1.6

 Bootstrap: shub
-From: mkandes/ubuntu:ubuntu-18.04
+From: mkandes/naked-singularity:ubuntu-18.04
 
 %labels
 
@@ -10,7 +10,7 @@ From: mkandes/ubuntu:ubuntu-18.04
     AUTHOR_NAME Marty Kandes
     AUTHOR_EMAIL mkandes@sdsc.edu
 
-    LAST_UPDATED 20200809
+    LAST_UPDATED 20201226
 
 %setup