Use a lighter PID 1 than $SHELL

When used as PID 1, /bin/sh takes 3248 kB compared to the 4136 kB
taken by /bin/bash. It's not a lot, but is memory that can be saved
for free.

As a nice side-effect, this unbreaks 'create' with podman-0.9.1 because
'podman create ...' doesn't work without a command.

containers/podman#1452

fedora-toolbox

@@ -97,7 +97,8 @@ create()
                  --tty \
                  --volume $HOME:$HOME \
                  --volume $XDG_RUNTIME_DIR:$XDG_RUNTIME_DIR \
-                 $toolbox_image >/dev/null 2>&42; then
+                 $toolbox_image \
+                 /bin/sh >/dev/null 2>&42; then
         echo "$0: failed to create container $toolbox_container"
         exit 1
     fi