Prefer to always create containers when running with Docker Executor

Closes #2347

See merge request gitlab-org/gitlab-runner!818

(cherry picked from commit 1c0fe99d)

f5387761 Prefer to always create containers when running with Docker Executor
c0fb1f54 Hook to previous build container instead of hooking to cache containers. Cache…
7d772715 Fix specs
a140a800 Fix unit tests failures
3ace2dde Remove unneeded vendor changes
43685ea3 Improve tests
effb6bbb Add dynamic cache container to a list of temporary caches
3aca6259 Refactor executor_docker to reduce method complexity
38ff2e46 Fix gofmt
fab6bee3 explicit nil return