add a documentation note on backslash usage in shell form of RUN
add a documentation note on backslash usage in shell form of RUN

Signed-off-by: Amen Belayneh <0bb20058ed34884ae836e6b6e36bab0a4d8a2ec3@gmail.com>

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