Run in Docker documentation lacks context

[codevbus]

Actual behavior
The documentation for running Kaniko in a Docker container via run_in_docker.sh lacks appropriate context. The script does not appear to function correctly if a remote build context is specified, or if a Dockerfile is referenced outside the container filesystem:

~/go/src/github.com/GoogleContainerTools/kaniko/run_in_docker.sh /Users/mike/projects/kaniko/Dockerfile gs://gc-bucket/context.tar.gz gcr.io/project-id/nginx-alpine

docker: Error response from daemon: invalid mode: /workspace.

Expected behavior
As described in the documentation, this should be a valid command string:

./run_in_docker.sh <path to Dockerfile> <path to build context> <destination of final image>

To Reproduce
Steps to reproduce the behavior:

1. Run run_in_docker.sh using a local Dockerfile and remote build context.

Additional Information

• Dockerfile

FROM nginx:alpine
WORKDIR /root/
COPY www /usr/share/nginx/html

• Build Context

COPY www /usr/share/nginx/html

• Kaniko Image (fully qualified with digest)
  latest

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

[codevbus]

PR in #829