Dockerfile path can no longer be an HTTP(S) address

[willcro]

Actual behavior
Kaniko attempts to resolve any URL as a relative path. This causes an error since the directory doesn't exist.

Expected behavior
If a URL is passed as the --dockerfile argument, it should make a GET request to fetch the Dockerfile. The downloaded Dockerfile should then be used for the build.

To Reproduce
Steps to reproduce the behavior:

1. kaniko --context . --dockerfile https://example.com/dockerfile --no-push
2. error building image: reading dockerfile at path /https:/example.com/dockerfile: open /https:/example.com/dockerfile: no such file or directory

Additional Information

• This appears to have been broken by Resolve relative paths to absolute paths in command line arguments #736
• Tested with gcr.io/kaniko-project/executor:de093f9e05c5ddd11aee185cabecc3249bd090ec

Triage Notes for the Maintainers

Description	Yes/No
Please check if this a new feature you are proposing
Please check if the build works in docker but not in kaniko
Please check if this error is seen when you use --cache flag
Please check if your dockerfile is a multistage dockerfile

[tejal29]

@willcro Looks like a bug here
https://github.com/GoogleContainerTools/kaniko/blob/master/cmd/executor/cmd/root.go#L249,
Fixed in #804