Change image provision directory to "images"

Previous name (before #1641) was "container." New name (from #1641) was "image"
but a plural name seems better.

Signed-off-by: Jonathon Anderson <janderson@ciq.com>

internal/app/wwctl/image/copy/main_test.go

@@ -28,6 +28,6 @@ func Test_Copy(t *testing.T) {
 		baseCmd.SetArgs([]string{"-b", "test-image", "test-image-copy"})
 		err := baseCmd.Execute()
 		assert.NoError(t, err)
-		assert.FileExists(t, path.Join(env.BaseDir, testenv.WWProvisiondir, "image", "test-image-copy.img"))
+		assert.FileExists(t, path.Join(env.BaseDir, testenv.WWProvisiondir, "images", "test-image-copy.img"))
 	})
 }