Support parsing of the tag even a digest is provided

[rcjsuen]

FROM alpine:non-existent-tag@sha256:1fd62556954250bac80d601a196bb7fd480ceba7c10e94dd8fd4c6d1c08783d5

$ docker build .
[+] Building 0.1s (5/5) FINISHED                                                                                                                                                                               docker:default
 => [internal] load build definition from Dockerfile                                                                                                                                                                     0.0s
 => => transferring dockerfile: 138B                                                                                                                                                                                     0.0s
 => [internal] load .dockerignore                                                                                                                                                                                        0.0s
 => => transferring context: 2B                                                                                                                                                                                          0.0s
 => [internal] load metadata for docker.io/library/alpine:non-existent-tag@sha256:1fd62556954250bac80d601a196bb7fd480ceba7c10e94dd8fd4c6d1c08783d5                                                                       0.0s
 => CACHED [1/1] FROM docker.io/library/alpine:non-existent-tag@sha256:1fd62556954250bac80d601a196bb7fd480ceba7c10e94dd8fd4c6d1c08783d5                                                                                  0.0s
 => exporting to image                                                                                                                                                                                                   0.0s
 => => exporting layers                                                                                                                                                                                                  0.0s
 => => writing image sha256:5a2ff9dc5af00d30bd1864bc77b5d83b54f1d06bcee20d99732f6fa72fa43b00

We should return the range of non-existent-tag instead of returning null.

const lib = require("dockerfile-ast");
const instruction = lib.DockerfileParser.parse("FROM alpine:non-existent-tag@sha256:1fd62556954250bac80d601a196bb7fd480ceba7c10e94dd8fd4c6d1c08783d5").getInstructions()[0];
console.log(instruction.getImageTagRange());

null