Rename container_output() to attach(), removing old attach()

The previous version of `attach` didn't look like it was doing
anything useful, as it just yielded 4096-byte chunks of raw /attach
output, without doing any frame parsing.

Now `attach` is identical to `logs`, except that by default it
doesn't show historical output. `logs` is now a wrapper around
`attach` which sets `logs=True`, and its behaviour is unchanged.