fix(docker): Unintentional unattended dry runs

[PR feedback](#1321 (comment))
prompted me to reconsider having a default command at all. We should assume,
unfortunately, that many new users will just skim the docs enough to enable the image's
cron integration but not actually incrementally test their configuration. In those
cases, they'd end up sending dry-run non-download requests for all their subscriptions
every 6 hours for no good reason. There's just no way to provide a default command that
isn't also providing a footgun.

Author: rpatterson

docker/root/defaults/cron

@@ -4,9 +4,12 @@
 # default `./config.yaml` and `./subscriptions.yaml`, so you don't need to use the
 # `--config` CLI option or pass a `SUBPATH` to the `$ ytdl-sub sub` sub-command.
 #
-# To prevent users accidentally triggering throttles or bans or downloading before
-# testing their configuration, these default options only simulate a few
-# downloads. Remove the `--dry-run` and `-o ...` CLI options when you've tested your
-# configuration and you're ready to download entries unattended:
-
-ytdl-sub --dry-run sub -o '--ytdl_options.max_downloads 3'
+# Test your configuration and subscriptions carefully before automating downloads to
+# prevent triggering throttles or bans:
+#
+# https://ytdl-sub.readthedocs.io/en/latest/guides/getting_started/downloading.html
+#
+# Once you've tested your configuration and you're ready to download entries unattended,
+# remove the next line and un-comment the following line:
+echo "WARNING: Read /config/ytdl-sub-configs/cron and modify to automate downloads."
+# ytdl-sub sub