build.md: Document --build-arg without value
build.md: Document --build-arg without value

This use case is currently _working correctly_, which is nice, but there is no documentation to be found about it. This PR fixes that.

Signed-off-by: Per Lundberg <71d64ded6fa579d17550ad9d286fc776c741ceaa@gmail.com>

docker/cli,docker/cli,thaJeztah/cli,cpuguy83/cli,cpuguy83/cli,thaJeztah/cli