If curl is not available to download the docker install script, try
with wget instead.
If neither is available or both fail, return a simple failing script
so that we don't carry on regardless.
Fixes: https://github.com/basecamp/kamal/issues/395
To avoid polluting the default SSH directory with lots of Kamal config,
we'll default to putting them in a `kamal` sub directory.
But also make the directory configurable with the `run_directory` key,
so for example you can set it as `/var/run/kamal/`
The directory is created during bootstrap or before any command that
will need to access a file.
