Validate env var names before writing env var files

[fg-j]

Summary

In the process of using Netflix/go-env to log build configuration, I uncovered this bug. I'm hoping to fix it upstream, but have to go through some bureaucracy to do it. For now, I've added validation to the FormattedMap constructor that should help out with this problem.

This has also made me wonder whether we should have some validation in the packit.Environment() functions, since right now a user can easily add invalid env var values to the environment. I'm not sure how the lifecycle handles that situation today.

❓ @paketo-buildpacks/tooling-maintainers , do you think we should add validation to the packit.Environment methods?

Use Cases

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[ryanmoran]

Are we worried about it being less clear what is happening here since this code now just silently ignores bad inputs?

[fg-j]

@ryanmoran You're right. I think it's confusing. I'll get around the bug in go-env a different way. But I do think some validation of environment variables could be useful.

I just played around with using packit.Environment to set environment variables with names that don't comply with the regex validation (e.g. setting DOTNET ROOT instead of DOTNET_ROOT or RUNTIME=VERSION instead of RUNTIME_VERSION. These get loaded into the environment without error, but evaluating them results in unexpected behaviour. For example, in a launch-time container,

cnb@5f362f11abd6:/workspace$ env
...
...
DOTNET_=ROOT=/workspace/.dotnet_root
...
...
cnb@5f362f11abd6:/workspace$ echo $DOTNET_=ROOT
ROOT=/workspace/.dotnet_root=ROOT

For that reason, I think the validation would make more sense as part of the EnvironmentWriter(). If that sounds reasonable, I'll shift the validation in this PR.