Implemented a FakeStorage alternative that supports spinning
up a remote container on DOCKER_TEST_HOST to serve files over
an offline-compiled Go static web server image so that tests which
use URLs in Dockerfile can build them over at the daemon side.
`fakeStorage` function now automatically chooses if it should
use a local httptest.Server or a remote container.
This fixes the following tests when running against a remote
daemon:
- `TestBuildCacheADD`
- `TestBuildCopyWildcardNoFind`
- `TestBuildCopyWildcardCache`
- `TestBuildADDRemoteFileWithCache`
- `TestBuildADDRemoteFileWithoutCache`
- `TestBuildADDRemoteFileMTime`
- `TestBuildADDLocalAndRemoteFilesWithCache`
- `TestBuildADDLocalAndRemoteFilesWithoutCache`
- `TestBuildFromURLWithF`
- `TestBuildApiDockerFileRemote`
Signed-off-by: Ahmet Alp Balkan <ahmetalpbalkan@gmail.com>
