Allow users to provide a custom gradle.properties file

[dmikusa]

Summary

When a binding of type or kind equals gradle is present, the contents of that file are copied to $GRADLE_USER_HOME/gradle.properties. This is picked up by Gradle when it runs and allows the user to pass additional configuration to Gradle. For example, this can be used for providing credentials to a remote Maven repository.

Use Cases

Resolves #23

Checklist

• I have viewed, signed, and submitted the Contributor License Agreement.
• I have linked issue(s) that this PR should close using keywords or the Github UI (See docs)
• I have added an integration test, if necessary.
• I have reviewed the styleguide for guidance on my code quality.

[dmikusa]

A couple of notes on this PR:

1. Gradle doesn't have a property similar to Maven's where you can point it directly to a configuration file. You can set a property that changes the $GRADLE_USER_HOME location, but that is not the same.
2. If we use a system property or env variable to change $GRADLE_USER_HOME, that causes issues with caching of Gradle downloaded resources, which also live under this directory. We should probably document these settings as under the control of the buildpack & off-limits to users setting them.
3. Rather than try to move things around, I just move the config file into $GRADLE_USER_HOME/gradle.properties. This should be safe because that directory is created by the build pack and there should be no user config in that location (the user can only put a gradle.properties file into their application.

[ekcasey]

I think we need to add a hash of the properties file to the application layer metadata. Right now a change to the properties file won't trigger a rebuild of the app layer.

[dmikusa]

I think we need to add a hash of the properties file to the application layer metadata. Right now a change to the properties file won't trigger a rebuild of the app layer.

I added this. It stores the sha256 hash of the properties into the metadata now, similar to what we do in the maven cnb with settings.xml.