When running arbitrary commands in the container, the default configuration and
subscription file paths don't work. I've tested using `$ docker compose up
--workdir="/config" ...` and the s6-overlay base image bits seem to work just fine, the
output from running cron jobs appears. Then running `$ ytdl-sub sub` without options
"Just Works", so I think baking a `WORKDIR` into the image should be OK, but this should
be tested further before breaking everybody's deployments.
I tested locally by bringing up the compose services and confirming from the output that
the s6-overlay init and cron seem to be working correctly. Then I ran `$ ytdl-sub sub
--dry-run` using both `$ docker compose run ...` and `$ docker compose exec ...`. Before
this change, it exits immediately with a message about the subscriptions file
missing. After this change, it starts to download the `NOVA PBS` subscription. In the
GUI image, I also tested the same command in the VS Code web UI terminal.
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