feat: renovate docker tag major/minor versions (#1029)

This feature enables docker tag "upgrading". Examples:
- From `node:6` to `node:8`
- From `node:6.10` to `node:6.11`
- From `node:6.11.1` to `node:6.11.3`
- From `node:6.10-onbuild` to `node:6.11-onbuild`
Currently these are disabled by default so they are opt-in, you can enable them by setting e.g. `{ docker: { minor: { enabled: true }, major: { enabled: true }}`

Closes #930