feat: add docker image prefix (#7164)

docs/usage/self-hosted-configuration.md

lib/config/common.ts

lib/config/definitions.ts

lib/util/exec/__snapshots__/exec.spec.ts.snap

lib/util/exec/common.ts

lib/util/exec/docker/index.ts

lib/util/exec/exec.spec.ts

lib/util/exec/index.ts