Add support for floki to interpret host environmental variables

[peter-oneill]

Alternative to PR for Tera-based implementation

Why this change?

This allows users to reference environmental variables in floki.yaml, for example to mount a $HOME subdirectory in a container as

docker-switches:
  - -v ${user_env:HOME}/.vim:/home/build/.vim

Any string matching \$\{user_env:([a-zA-Z0-9_]*)\} is captured by floki before deserialization, and the user's environmental variable name following user_env: is substituted in.

Relevant testing

• New UT added,
• Tested manually against a local floki.yaml file which successfully mounted a directory declared as ${user_env:HOME}, and printed a host environmental variable in the container in an init command.

Contributor notes

See Teams chat for idea discussion.

Checks

These aren't hard requirements, just guidelines

• New/modified Rust code formatted with cargo fmt
• Documentation and README.md updated for this change, if necessary
• CHANGELOG.md updated for this change

[bossmc]

We probably want to reserve ${<mode>:<value>} completely - so if we see -e ${lol:foo} we might want to throw an error rather than leaving it un-changed (this also allows adding additional things later e.g. -e KERNEL_VERSION=${exec:uname -r} without being a breaking change each time).

This also means that ${user_env:@*&} wants to be an error too - erroring when someone it looks like someone has tried but failed to use a feature is probably better than assuming they weren't aware of the feature and meant to type what they typed (might require an escape hatch for the incredibly rare case where someone wanted the un-expanded value)

[bossmc]

Interesting... there's a difference between an envvar being absent and being empty (though by default bash will expand $FOO to "" if FOO is unset - but see set -u and the various ${foo:bar}/%{foo:-bar} syntaxes).

I'm not sure what the right option is here, though I'd probably guess that erroring is more intuitive (the script expects to be using the value, if there is no value the script may well do the wrong thing if it's forced to treat it as "").

[maxdymond]

Fixed via use of tera