Use Docker bootstrap agent

Support for the library agent may have been removed in the migration
from singularity to apptainer. It may be restored later.

base_containers/pytorch/pytorch_cu.def.template

@@ -1,17 +1,17 @@
-BootStrap: library
-From: ubuntu:20.04
-
-%post
-    apt-get -y update
-
-    # Add universe repository (necessary for python3-pip)
-    apt-get -y install software-properties-common
-    add-apt-repository -y -u universe
-
-    # Install python packages
-    apt-get -y install python3 python3-pip
-
-    apt-get clean
-
-    # Install python dependencies
-    pip3 install --no-cache-dir torch==$torch_version torchvision==$torchvision_version torchaudio==$torchaudio_version -f $find_links
+Bootstrap: docker
+From: ubuntu:20.04
+
+%post
+    apt-get -y update
+
+    # Add universe repository (necessary for python3-pip)
+    apt-get -y install software-properties-common
+    add-apt-repository -y -u universe
+
+    # Install python packages
+    apt-get -y install python3 python3-pip
+
+    apt-get clean
+
+    # Install python dependencies
+    pip3 install --no-cache-dir torch==$torch_version torchvision==$torchvision_version torchaudio==$torchaudio_version -f $find_links