Use Vault for internal gitlab-runner configuration secrets
As an extension to the Vault proof of concept we want to consider how we can use Vault for secrets internal to the runner configuration.
For example, a user may not want to save the S3 or other credentials in plain text in the
config.toml and thus may want to have those secrets fetched from Vault by the runner, which will authenticate using one of the known methods.
Secrets to consider
- GCS configuration
- S3 configuration for cache
- All docker+machine tokens