Variable default value should be considered instead of flagging it as an error

[rcjsuen]

FROM ${VAR:-alpine}

We should not be flagging the variable as an error as -alpine means that alpine should be swapped in if the variable is not set.

$ dockerfile-utils lint Dockerfile.2
Line: 1
FROM ${VAR:-alpine}
     ^^^^^^^^^^^^^^
Error: base name (${VAR:-alpine}) should not be blank

$ docker build -f Dockerfile.2 .
[+] Building 2.1s (6/6) FINISHED                                                                                    docker:desktop-linux
 => [internal] load build definition from Dockerfile.2                                                                              0.0s
 => => transferring dockerfile: 59B                                                                                                 0.0s
 => [internal] load metadata for docker.io/library/alpine:latest                                                                    1.0s
 => [auth] library/alpine:pull token for registry-1.docker.io                                                                       0.0s
 => [internal] load .dockerignore                                                                                                   0.0s
 => => transferring context: 2B                                                                                                     0.0s
 => [1/1] FROM docker.io/library/alpine:latest@sha256:77726ef6b57ddf65bb551896826ec38bc3e53f75cdde31354fbffb4f25238ebd              1.0s
 => => resolve docker.io/library/alpine:latest@sha256:77726ef6b57ddf65bb551896826ec38bc3e53f75cdde31354fbffb4f25238ebd              0.0s
 => => sha256:77726ef6b57ddf65bb551896826ec38bc3e53f75cdde31354fbffb4f25238ebd 1.85kB / 1.85kB                                      0.0s
 => => sha256:216266c86fc4dcef5619930bd394245824c2af52fd21ba7c6fa0e618657d4c3b 528B / 528B                                          0.0s
 => => sha256:1d34ffeaf190be23d3de5a8de0a436676b758f48f835c3a2d4768b798c15a7f1 1.47kB / 1.47kB                                      0.0s
 => => sha256:d25f557d7f31bf7acfac935859b5153da41d13c41f2b468d16f729a5b883634f 3.62MB / 3.62MB                                      0.4s
 => => extracting sha256:d25f557d7f31bf7acfac935859b5153da41d13c41f2b468d16f729a5b883634f                                           0.3s
 => exporting to image                                                                                                              0.0s
 => => exporting layers                                                                                                             0.0s
 => => writing image sha256:b4627cf16b569c0672269d26e194453a38cc283185da70f6214f6f23bfc607e4                                        0.0s

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