Support new --start-interval flag in HEALTHCHECK

[rcjsuen]

This feature is documented but not released. It is supposedly targeted for 25.0.0 according to docker/cli#4486.

FROM alpine
HEALTHCHECK --start-interval=5s CMD ls

docker build .
[+] Building 0.1s (2/2) FINISHED                                                                docker:default
 => [internal] load build definition from Dockerfile                                                      0.0s
 => => transferring dockerfile: 88B                                                                       0.0s
 => [internal] load .dockerignore                                                                         0.0s
 => => transferring context: 2B                                                                           0.0s
Dockerfile:2
--------------------
   1 |     FROM alpine
   2 | >>> HEALTHCHECK --start-interval=5s CMD ls
   3 |
--------------------
ERROR: failed to solve: dockerfile parse error on line 2: unknown flag: start-interval

The feature can be enabled with syntax=docker/dockerfile:1. So we should still support it.

#syntax=docker/dockerfile:1
FROM alpine
HEALTHCHECK --start-interval=5s CMD ls

$ dockerfile-utils lint
Line: 3
HEALTHCHECK --start-interval=5s CMD ls
            ^^^^^^^^^^^^^^^^
Error: Unknown flag: start-interval