Changes

jobs/{master,gerrit}: set ulimit filesize to 1 GiB

Set a max file size limit of 1 GiB via ulimit. This prevents jobs from
filling up the disk by generating one huge log file.

I have chosen 1 GiB as limit, because jobs that clone linux kernel
repos may have packed git artifacts that are several 100 MiB in size.

It would be even better to set the overall max disk space that can be
used from one "docker run" call, but this is currently not supported for
ext4: https://github.com/moby/moby/issues/29364

Related: OS#6704
Change-Id: I4f9d5ad33c2d6e003d239e8b2c97753d64dd00f6

jobs: manually trigger "git gc" for linux.git

We already use shallow clones for linux.git, and git runs a garbage
collector automatically to ensure no longer needed artifacts are
removed. But this did not work anymore on build4 for
build-kernel-net-next, and it increased up to 8.4 GiB:

    8,4 GiB /build-kernel-net-next

After running "git gc" again:

    3,3 GiB /build-kernel-net-next

The reason was that "git gc" did not exit successfully once and wrote a
log file, then refused to run again. Run "git gc --auto" explicitly in
foreground to let abort in such a case, so it does not go unnoticed in
the future.

Change-Id: Ic81ae5a1edb0dbe119e06cc3b49ea69ef81b2a5f

scripts/docker-cleanup: add "docker system prune"

On build4 this freed up another 8.223GB when I just ran it:

  $ docker system prune
  WARNING! This will remove:
    - all stopped containers
    - all networks not used by at least one container
    - all dangling images
    - all dangling build cache

  Are you sure you want to continue? [y/N]

Change-Id: I98aa5324eaf7cf319625d85bc80d431b89ee50a7