Add variable expansion to root_share and debug services

[xiota]

Uses os.path.expandvars() to expand variables in root_share and debug services.

Tested with $HOME variable and non-variable $TEST. Existing configs containing $ will not break as long as the word does not correspond to a variable.

Closes #61

[rusty-snake]

Maybe expanduser (i.e. ~ -> /home/user) would be somethning some users expect too.

It's always hard to explain that $HOME and ~ are shell things. And that they are different things.

[igo95862]

Hello @xiota

I don't like not having explicit control over how inputs are mutated.

One idea that might be interesting is changing behavior based on if raw_bwrap_args is a string or a list on the services file. The lists would work as is but strings would be treated as shell commands.

For example,

raw_bwrap_args='''
--setenv X "$Y"
'''

This uses the TOML multi line string syntax and would be shlex split and variables would be expanded.

By the way is there a particular reason you would want the environment variables to be used? Does your $HOME change a lot?

[xiota]

interpreting variables for arguments is fairly common. It increases configuration flexibility while decreasing implementation complexity. Separate home share and root share are no longer needed. Complicated workarounds to add functionality are no longer needed. Instances can be portable. They can be used by different users on different computers.

Compatibility with existing configurations should be high because the use of $ in paths and arguments on Linux/Unix is uncommon.