Improve error message when environment map contains null values

[oliver-brm]

Environment:

• Jib version: 3.2.1
• Build tool: Gradle 7.4.1
• OS: N/A

Description of the issue:

Jib's checks if any of the keys and values set in the container.environment configuration is null, which is super helpful. However, the error message when an container.environment value is null currently reads only like this: "environment map contains null values". For someone not so deep into Jib, this could be not enough of a hint to the actual problem.

Expected behavior:
The error message should contain the offending keys, for example like so: "environment map contains null values for key(s): FOO, BAR, BAZ"

Steps to reproduce:

1. Use an environment map with a value set to null
2. Run jib (e.g. ./gradlew jib)

I can offer a solution via PR.

[elefeint]

Thanks, this seems like a useful improvement.

[mpeddada1]

Fixed with #3672

[mpeddada1]

Thank you for your contribution! jib-maven-plugin and jib-gradle-plugin 3.3.0, jib-core 0.22.0 and jib-cli 0.11.0 have been released with this fix.