ModifiableInstruction's toString() does not include the flags

[rcjsuen]

We are only including the arguments and not the flags.

import { DockerfileParser } from 'dockerfile-ast';
let dockerfile = DockerfileParser.parse(
    "HEALTHCHECK --interval=30s CMD echo"
);
let instruction = dockerfile.getInstructions()[0];
console.log(instruction.toString());

HEALTHCHECK CMD echo