Add Dinesh Subhraveti to MAINTAINERS

Docker-DCO-1.1-Signed-off-by: Dinesh Subhraveti <dineshs@altiscale.com> (github: dineshs-altiscale)
Merge pull request #6060 from dineshs-altiscale/maintainers

Add Dinesh Subhraveti to MAINTAINERS
Added highlight for the COPY instruction

Docker-DCO-1.1-Signed-off-by: Asbjorn Enge <asbjorn@hanafjedle.net> (github: asbjornenge)