Jib version 1.5.0 released

[Chanseok Oh]

Version 1.5.0 of jib-maven-plugin and jib-gradle-plugin has been released with new features and fixes.

Major Changes:

• Can now set file timestamps (last modified time) in the image with `container.filesModificationTime`. The value should either be `EPOCH_PLUS_SECOND` to set the timestamps to Epoch + 1 second (default behavior), or an ISO 8601 date time parsable with [`DateTimeFormatter.ISO_DATE_TIME`](https://docs.oracle.com/en/java/javase/11/docs/api/java.base/java/time/format/DateTimeFormatter.html) such as `2019-07-15T10:15:30+09:00` or `2011-12-03T22:42:05Z`
• Can now set container creation timestamp with `container.creationTime`. The value should be `EPOCH`, `USE_CURRENT_TIMESTAMP`, or an ISO 8601 date time
• For Google Container Registry (gcr.io), Jib now tries [Google Application Default Credentials](https://developers.google.com/identity/protocols/application-default-credentials) (ADC) last when no credentials can be retrieved. ADC are available on many Google Cloud Platform (GCP) environments (such as Google Cloud Build, Google Compute Engine, Google Kubernetes Engine, and Google App Engine). Application Default Credentials can also be configured with `gcloud auth application-default login` locally or through the `GOOGLE_APPLICATION_CREDENTIALS` environment variable.
• When building to a registry, Jib now skips downloading and caching base image layers that already exist in the target registry. This feature will be particularly useful in CI/CD environments. However, if you want to force caching base image layers locally, set the system property `-Djib.alwaysCacheBaseImage=true`
• `container.useCurrentTimestamp` has been deprecated in favor of `container.creationTime` with `USE_CURRENT_TIMESTAMP`

See CHANGELOG.md (Maven / Gradle) for more details.