Proposal: ENVFILE support in builder (#7688)

I implemented a feature discussed in #7688.
```
$ cat Dockerfile
FROM busybox
ENVFILE foo.txt
CMD echo $FOO $BAR
```
```
$ cat foo.txt
FOO=42
BAR=72
```
Small modification for `ParseEnvFile` function in `github.com/docker/docker/opts/envfile` was applied to support URL(remote file).
(So usage like `docker run --env-file="http://www.example.com/envfile"` is now possible)
I added some unit tests for ENVFILE as sample use cases.