remove existing containers when doing image pull

In the case where there is a new container image available or if
someone switches the container image they are using, there may be an
existing container created on disk. In this case, we should remove the
existing container, so the user enters a freshly created container
from the new container image.
diff --git a/rhcos-toolbox b/rhcos-toolbox
index bca330a..ac6ce55 100755
--- a/rhcos-toolbox
+++ b/rhcos-toolbox
@@ -22,11 +22,17 @@
 run() {
     if ! image_exists; then
         image_pull
+        if container_exists; then
+            sudo podman rm "${TOOLBOX_NAME}"
+        fi
     elif ! image_fresh; then
         read -r -p "There is a newer version of ${TOOLBOX_IMAGE} available. Would you like to pull it? [y/N] "
 
         if [[ $REPLY =~ ^([Yy][Ee][Ss]|[Yy])+$ ]]; then
             image_pull
+            if container_exists; then
+                sudo podman rm "${TOOLBOX_NAME}"
+            fi
         else
             echo "Skipping retrieval of new image.."
         fi