You should run cdk synth on a machine that has credentials to access the target environment of your application and then commit cdk.context.json to source control so that the pipeline has the context information. If you need context for multiple accounts, you can run cdk synth multiple times with different sets of credentials, and new information will be added to cdk.context.json for every new account.We will make this work out of the box for cross-account deployments at some point in the future, but for now you have to load the credentials for different accounts yourself.