sort lint warning output and make consistent

[daghack]

fixes #4844

This is a follow-up to #4761, and updates the printed source map for the lint warnings to be consistent with how they are displayed when viewing them via the docker --debug flag.

Old:

Lint Rule StageNameCasing
  Dockerfile:2
  Lint Rule 'StageNameCasing': Stage name 'buildER' should be lowercase (line 2)
  2  |  FROM $BASE:1.20-alpine AS buildER

New:

- Lint Rule 'StageNameCasing': Stage name 'buildER' should be lowercase (line 2)
Stage names should be lowercase
Dockerfile:2
--------------------
   1 |     ARG BASE=golang
   2 | >>> FROM $BASE:1.20-alpine AS buildER
   3 |
   4 |     WOrkdIR /go/src/github.com/restic/restic
--------------------

[daghack]

@tonistiigi or @crazy-max Merge? 😮 Thank you!