feat: Allow specifying Docker user (#4578)

docs/usage/self-hosted-configuration.md

lib/config/definitions.ts

lib/manager/bundler/artifacts.ts

lib/manager/cargo/artifacts.ts

lib/manager/common.ts

lib/manager/composer/artifacts.ts

lib/manager/gomod/artifacts.ts

lib/manager/gradle/index.ts

lib/manager/npm/post-update/npm.ts

lib/manager/npm/post-update/pnpm.ts

lib/manager/pipenv/artifacts.ts

lib/manager/poetry/artifacts.ts

renovate-schema.json