ci(buildkit): set timeout to image job

[crazy-max]

related to #4477 (comment)

Image build seems to hang and job timed out after 6h (default on GitHub Actions) for the past few days:

• https://github.com/moby/buildkit/actions/runs/7183260509/job/19562224050?pr=4460#step:13:464
• https://github.com/moby/buildkit/actions/runs/7163256834/job/19502241344?pr=4477#step:13:461

  time="2023-12-11T06:13:24Z" level=debug msg="diff applied" d=2.806054ms digest="sha256:1f5f54dd06e9d17c845fb7c5fdf944c3160d6c4a7a8c645ee8b08e5618c0e65a" media=application/vnd.oci.image.layer.v1.tar+gzip size=942 spanID=62556bbbe3ab4d04 traceID=ca85fd7fcf30de02e721e83aa9c1c2e5
  time="2023-12-11T06:13:24Z" level=debug msg="Using single walk diff for /tmp/buildkit-mount4051339310"
  time="2023-12-11T06:13:24Z" level=debug msg="> creating xiytdlg99ck50ouj0ghtlxhtm [/bin/syft-scanner]" span="[linux/riscv64] generating sbom using docker.io/docker/buildkit-syft-scanner:stable-1" spanID=b1b778eff0601a70 traceID=ca85fd7fcf30de02e721e83aa9c1c2e5

Unfortunately we don't have a stacktrace in BuildKit logs because the worklflow is canceled by the runner.

This change sets a timeout of 15 minutes and send SIGQUIT to the BuildKit container so we can get a stacktrace.