Portable OSS runners do not support GCP credentials for GCP IOs.

The situation here is that when a pipeline is run on a portable runner using a GCP IO, and uses docker for the SDK Harness environment, the SDK Harness does not have the user's GCP credentials available and the pipeline fails. There are apparently [pipeline options for setting credentials](https://github.com/apache/beam/blob/v2.33.0/sdks/java/extensions/google-cloud-platform-core/src/main/java/org/apache/beam/sdk/extensions/gcp/options/GcpOptions.java#L170), but as far as I can tell they are either meant only for non-portable pipelines, or only for the Dataflow runner.

The tricky part of implementing this is that credentials for GCP are not straightforward, and having them available for something like the Application Default Credentials API involves copying over multiple files or environment variables. The following article provides a lot of context for the difficulties involved: [https://medium.com/datamindedbe/application-default-credentials-477879e31cb5](https://medium.com/datamindedbe/application-default-credentials-477879e31cb5)

Possible solutions. Note these are mostly untested:
- Perform some volume-mounting when calling the "docker run" command to mount directories containing credentials. Preferably this can be set via some sort of pipeline option. (This could potentially also be used to provide directories for docker containers to write output files to with TextIO or FileIO.) See the article above for an example.
** This solution may not work with runners on remote endpoints though. The directory mounted must be on the same machine as the docker container to work properly, which may not be possible in some cases with remote runners.
- Require custom containers with appropriate credentials provided. This is more robust than the solution above, but less user-friendly, and would require a good amount of documentation to be available.
** This could be possible in conjunction with the solution above, and might be a good way of supporting GCP credentials on remote runners. Custom containers can store any valid credentials of the user's choice, (for example service account credentials for a production service) and then be run on any machine.

Imported from Jira [BEAM-13215](https://issues.apache.org/jira/browse/BEAM-13215). Original Jira may contain additional context.
Reported by: danoliveira.