Touch up "Dockerfile reference"
Touch up "Dockerfile reference"

- add example for `docker build -f ...`
- modifiy `FROM` explaination into bullet points
- add/clarify examples for `LABEL` and `ADD` instructions
- fix review comments (PR #16109)

Signed-off-by: Charles Chan <472c3ebfa984240a364fc10fa7feeab616478c19@users.noreply.github.com>

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