DOCS_RS_LOCAL_DOCKER_IMAGE seems to be depricated

[Ayush]

When using the default instructions to add packages for docs.rs, the instructions seem to contain the environment variable DOCS_RS_LOCAL_DOCKER_IMAGE. This however gives the following error:

env variable DOCS_RS_LOCAL_DOCKER_IMAGE is no longer accepted; use DOCSRS_DOCKER_IMAGE instead

I am going to open a pull request soon but just wanted to open the issue for information about this.