Adds support for custom docker registries. To work (for now), registries must support anonymous public access to their v2 API. Tested against quay.io and gcr.io, including tags pagination for quay. Also needed to add a 10s timeout for registry queries to catch private/firewalled registries that we can't access.

Closes #797

* chore(deps): update dependency prettier to v1.9.0

* run prettier

This reverts commit 371f1cbf.

Use got’s built-in caching with in-memory map.

Closes #1028

* chore(deps): update dependency prettier to v1.8.0

* reformat

This PR adds support for 'AS' names in Dockerfiles. e.g. `FROM node:8 AS base`. It also adds logic to detect and ignore - for now - any image sources from custom registries.

docker and npm

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

* chore(deps): update dependency eslint-config-airbnb-base to v12.0.0

* fix lint

This feature adds initial support for renovating Dockerfiles. Renovate now:
- Detects all `Dockerfile`s in repo
- Searches for `FROM x` in first non-comment line, breaks x into image, tag, digest
- Queries public Docker registry for image:tag combination to find latest digest
- Patches Dockerfile if necessary
- Creates branches/PRs as like with npm

Closes #795