gistfile1.txt

In container:
/tmp/f1
/tmp/f2
/tmp/dir/f1
/tmp/dir/f2

CMD: docker cp container:source target

Source		Target		Body (tar unless otherwise indicated)		Stat Hdr	Result
/tmp		.		tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	./tmp/...
/tmp		./		tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	./tmp/...
/tmp		dir(missing)	tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	./dir/f1, dir/f2, dir/dir/f1, dir/dir/f2
/tmp		dir(present)	tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	./dir/tmp/...
/tmp		dir/(missing)	tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	./dir/f1, dir/f2, dir/dir/f1, dir/dir/f2
/tmp		dir/(missing)	tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	./dir/tmp/...
/tmp		file(present)	tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	error
/tmp		file/(present)	tmp + tmp/f1, tmp/f2, tmp/dir/f1, tmp/dir/f2	tmp stat	error

I believe the key take-away from above is that the presence or lack of a / on target doesn't matter.
Presence or absent of the target indicates whether we're creating the target or coping into the target.
I think we can determine what we're doing to do based just on the target info and the stat header (which includes a isDir flag)

Source		Target		Body (tar unless otherwise indicated)		Stat Hdr	Result
/tmp/f1		dir(present)	f1 (note, no tmp info) - not a tar		f1 stat		./dir/f1
/tmp/f1		dir/(present)	f1 - not a tar					f1 stat		./dir/f1
/tmp/f*		dir		f1, f2						none		./dir/f1, dir/f2
/tmp/f1		file(present)	f1 - not a tar					f1 stat		./dir/file
/tmp/f1		file(missing)	f1 - not a tar					f1 stat		./dir/file
/tmp/f*		file(present)	f1, f2						none		error file not a dir
/tmp/f*		file(missing)	f1, f2						none		error file not a dir
/tmp/*		dir(present)	f1, f2, dir/f1, dir/f2				none		./dir/f1, dir/f2, dir/dir/f1, dir/dir/f2

I think we can determine what to do in each case by only looking at what on the CLI and checking the stat header.
"none" stat header means we have more than one file in the tar.

Copying a single target that is a dir is still "one target" - meaning we have a stat header for the 
dir and the tar contains lots of files.

Net:
A docker tar (tar w/o stat header) means we have more than one src, so target MUST be a dir (present).
A single src (file or dir) means we create target (if not there) and put src into it (a rename of top-most thing).
A single src (file or dir) means we copy into target (if a dir) and put src into dir (copy not a rename).
A single src (dir) means we copy error if target is a file.
A single src (file) means we copy (replace) target if its a file.