Secrets should be interpolated at runtime so we do want the file in git.
But add a warning at the top to avoid adding secrets or git ignore the
file if you do.
Also provide examples of the three options for interpolating secrets.
Rather than redirecting the global $stdout, which is not never clever in
a threaded program, we'll make the secrets commands aware they are
being inlined, so they return the value instead of printing it.
Additionally we no longer need to interrupt the parent process on error
as we've inlined the command - exit 1 is enough.
Add env files back in for secrets - hides them from process lists and
allows you to pick up the latest env file when running
`kamal app exec` without reusing.
If you can have an alias like:
```
aliases:
rails: app exec -p rails
```
Then `kamal rails db:migrate:status` will execute
`kamal app exec -p rails db:migrate:status`.
So this works, we'll allow multiple arguments `app exec` and
`server exec` to accept multiple arguments.
The arguments are combined by simply joining them with a space. This
means that these are equivalent:
```
kamal app exec -p rails db:migrate:status
kamal app exec -p "rails db:migrate:status"
```
If you want to pass an argument with spaces, you'll need to quote it:
```
kamal app exec -p "git commit -am \"My comment\""
kamal app exec -p git commit -am "\"My comment\""
```
Donal McBreen