Use smaller test image - Githubissues

Description

Switched to a smaller Alpine testing image that works with BATS and doesn't require use of digests to lock.

Checklist

[X] Chart version bumped in Chart.yaml
[X] Any new variables have documentation and examples in values.yaml, even if commented out
[X] Any new variables added to the README.md
[X] Squash into a single commit (or explain why you'd prefer not to), except...
[X] ...additional commit added for CHANGELOG.md entry
[X] Helm lint + tests passing?

gocd / helm-chart