Adds 'WARNING' prefix to the rule check output

[daghack]

Old Output

[+] Building 0.1s (2/2) FINISHED                                                                                                         docker-container:dev
 => [internal] connecting to local controller                                                                                                            0.0s
 => [internal] load build definition from Dockerfile                                                                                                     0.0s
 => => transferring dockerfile: 171B                                                                                                                     0.0s
FromAsCasing - https://docs.docker.com/go/dockerfile/rule/from-as-casing/
The 'as' keyword should match the case of the 'from' keyword
Dockerfile:1
--------------------
   1 | >>> FROM scratch as first
   2 |     COPY $app .
   3 |
--------------------

FromAsCasing - https://docs.docker.com/go/dockerfile/rule/from-as-casing/
The 'as' keyword should match the case of the 'from' keyword
Dockerfile:7
--------------------
   5 |     COPY $app .
   6 |
   7 | >>> FROM scratch asgard third
   8 |
--------------------

New Output

[+] Building 0.1s (2/2) FINISHED                                                                                                         docker-container:dev
 => [internal] connecting to local controller                                                                                                            0.0s
 => [internal] load build definition from Dockerfile                                                                                                     0.0s
 => => transferring dockerfile: 171B                                                                                                                     0.0s

WARNING: FromAsCasing - https://docs.docker.com/go/dockerfile/rule/from-as-casing/
The 'as' keyword should match the case of the 'from' keyword
Dockerfile:1
--------------------
   1 | >>> FROM scratch as first
   2 |     COPY $app .
   3 |
--------------------

WARNING: FromAsCasing - https://docs.docker.com/go/dockerfile/rule/from-as-casing/
The 'as' keyword should match the case of the 'from' keyword
Dockerfile:7
--------------------
   5 |     COPY $app .
   6 |
   7 | >>> FROM scratch asgard third
   8 |
--------------------

[daghack]

@tonistiigi Can I get a merge as well?