docker-build man page doesn't reflect new rules for Dockerfile path

[starthal]

Description

Now that #886 allows the Dockerfile to be outside the build context, the man page is no longer correct.

Describe the results you received:

The docker-build man page says for --file:

If the path is a relative path and you are building from a local directory, then the path must be relative to that directory. If you are building from a remote URL pointing to either a tarball or a Git repository, then the path must be relative to the root of the remote context. In all cases, the file must be within the build context. The default is Dockerfile.

Describe the results you expected:

This should reflect the actual behavior. If I understand correctly, the path is actually relative to the directory in which docker build was run, not the build context dir specified in PATH. I don't know the rules for remote URLs.

Output of docker version:

Docker version 18.09.0, build 4d60db4

[starthal]

Looking through the man page, it also contains (emphasis mine)

This will read the Dockerfile from the directory specified in PATH. It also sends any other files and directories found in the current directory to the Docker daemon. The contents of this directory would be used by ADD commands found within the Dockerfile.

Which seems to conflict with the current docs:

The docker build command builds an image from a Dockerfile and a context. The build’s context is the set of files at a specified location PATH or URL. The PATH is a directory on your local filesystem. The URL is a Git repository location.