tricks.md

Tips and Tricks

Building a layer from scratch

There are a couple of cases where it may be useful to build a layer from scratch. For example to derive a new base install of an OS or to build a "tarball" type image which just carries data and will not actually be run by a container runtime.

The way to accomplish this in stacker is to use a build only layer (i.e. a layer that does not get emitted into the final OCI image, perhaps containing assets or something that will be used by the final container).

The best way to accomplish this is as follows:

build:
    from:
        type: docker
        url: docker://ubuntu:latest
    run: |
        touch /tmp/first
        touch /tmp/second
        tar -C /tmp -cv -f /contents.tar first second
    build_only: true
contents:
    from:
        type: tar
        url: stacker://build/contents.tar

Or e.g. to bootstrap a base layer for CentoOS 7:

build:
    from:
        type: docker
        url: docker://ubuntu:latest
    run: |
        yum -y --installroot=/rootfs --nogpgcheck install
        tar -C rootfs -zcf /rootfs.tar .
    build_only: true
contents:
    from:
        type: tar
        url: stacker://build/rootfs.tar

These work by creating the base for the system in a build container with all the utilities available needed to manipulate that base, and then asking stacker to create a layer based on this tarball, without actually running anything inside of the layer (which means e.g. absence of a shell or libc or whatever is fine).