Fix docs so WORKDIR mentions it works for COPY and ADD too
Fix docs so WORKDIR mentions it works for COPY and ADD too

The docs around COPY/ADD already mentioned that it will do a relative
copy/add based on WORKDIR, so that part is already ok.  Just needed to
tweak the WORKDIR section since w/o mentioning COPY/ADD it can be misleading.

Noticed by @phemmer

Signed-off-by: Doug Davis <ff3bde741799c0eff815b83a54eda3fc494df54f@us.ibm.com>

Zebrilee/cli,Zebrilee/cli,allencloud/cli,allencloud/cli,docker/cli,ashfall/cli,dnephin/cli,e11137/cli,docker/cli,thaJeztah/cli,ashfall/cli,riyazdf/cli,cpuguy83/cli,dnephin/cli,riyazdf/cli,e11137/cli,cpuguy83/cli,thaJeztah/cli,redpanda/cli,redpanda/cli