Add example of building a Dockerfile with local tar file build context

[njlr]

This PR adds an example of using a Dockerfile with a local tar as a build context.

This is not the ideal approach, which this is clearly signposted in the README!, but it shows an easy migration path from rules_docker. The existing example does not show how Bazel build artefacts can be passed to BuildX.

How it works:

1. Create a tar file using rules_pkg
2. Pipe the tar file into BuildX using sh_binary and run_binary
3. Use the BuildX output as a base for an oci_image

This is a replacement for container_run_and_commit.

The example could easily be adapted to behave like container_run_and_extract by changing the --output flag to a tar.

[thesayyn]

We have https://github.com/bazel-contrib/rules_oci/tree/main/examples/dockerfile, how is this different than that one?

[njlr]

We have https://github.com/bazel-contrib/rules_oci/tree/main/examples/dockerfile, how is this different than that one?

That example does not show how to use the output of other Bazel targets as input to the Dockerfile build

[thesayyn]

That example does not show how to use the output of other Bazel targets as input to the Dockerfile build

Than can we change it so it does? i don't see the difference between this example and the other one, other than putting the srcs into a tar file beforehand so you don't have to deal with paths that start with bazel-bin/cfg/bin

[njlr]

That example does not show how to use the output of other Bazel targets as input to the Dockerfile build

Than can we change it so it does? i don't see the difference between this example and the other one, other than putting the srcs into a tar file beforehand so you don't have to deal with paths that start with bazel-bin/cfg/bin

Yes, happy to do this.

The reason I made a new example is that it's a little more complicated than the existing example, so might hinder understanding.

What do you think?

[thesayyn]

We can add some comments explaining why there is two different buildx targets in the same package and the difference.

[njlr]

We can add some comments explaining why there is two different buildx targets in the same package and the difference.

I have merged the two examples

[thesayyn]

thanks!

[thesayyn]

There are some failures, can you take a look?

[thesayyn]

you need a ./app to fix the issue.

[njlr]

Thanks - this works

One thing I found a little surprising is that I need to add the srcs for both the mtree_spec and the tar.

Is this expected? Am I using it correctly?