refactor: use new-style docker management commands

This PR uses the new syntax for commands introduced in Docker 1.13, making it a bit easier to understand just what a particular command is trying to do.

This also pushes the long-form syntax for docker command flags, which are also easier to understand at a glance.

plugins/enter/subcommands/default

@@ -60,7 +60,7 @@ enter_default_cmd() {
   has_tty && local DOKKU_RUN_OPTS+=" -i -t"
   is_image_herokuish_based "$IMAGE" && local EXEC_CMD="/exec"
   # shellcheck disable=SC2086
-  "$DOCKER_BIN" exec $DOKKU_RUN_OPTS $ID $EXEC_CMD "${@:-$DOKKU_APP_SHELL}"
+  "$DOCKER_BIN" container exec $DOKKU_RUN_OPTS $ID $EXEC_CMD "${@:-$DOKKU_APP_SHELL}"
 }
 
 enter_default_cmd "$@"