Dockerfile: warn when trying to COPY/ADD using "~"

This is not really a bug, but this may be unexpected for users.

When using ~ as target for a COPY or ADD, the ~ is used as a literal character for the path.

For example:

touch somefile.txt

FROM alpine
COPY somefile.txt ~/somefile.txt

$ docker build -t foo .
$ docker run --rm foo sh -c 'find / -name somefile.txt'
/~/somefile.txt

Note that a literal ~ directory is created, and somefile.txt is copied into that directory.

Perhaps (once we support warnings / non-fatal errors) we can add a warning for this.

Start by running the Dockerfile example from the issue and confirm that COPY somefile.txt /somefile.txt creates //somefile.txt. Inspect the Dockerfile COPY/ADD handling and the project's warning support, then make the completed behavior warn about the literal ~ destination without making the build fail.