[8.14] build and push Docker image based on Chainguard base image (backport #13137)

[mergify]

Motivation/summary

Release two flavours of Docker images:

• the one we always do
• the one based on chainguard from Chainguard

Please note that we are going to preserve the current Dockerfile, so that users will still be able to build their own custom images based on ubuntu: this is needed because docker.elastic.co/wolfi/* is not a public base image, so docker build would fail for unauthenticated users.

Implementation details

I changed the Makefiles to support different Dockerfiles when running the docker build generation.
I added a tag for helping testing this in GitHub actions and then be able to push the docker images to an internal docker registry.
I fixed some warning regarding missing files when running the go build in the docker.

Checklist

• Update CHANGELOG.asciidoc
• Documentation has been updated

For functional changes, consider:

• Is it observable through the addition of either logging or metrics?
• Is its use being published in telemetry to enable product improvement?
• Have system tests been added to avoid regression?

How to test these changes

CI

Will create the docker images automatically when working on a feature branch or main/releases

docker images docker.elastic.co/observability-ci/apm-server-internal
REPOSITORY                                               TAG                     IMAGE ID       CREATED          SIZE
docker.elastic.co/observability-ci/apm-server-internal   8.15.0-SNAPSHOT-wolfi   dce4529d1383   11 minutes ago   126MB
docker.elastic.co/observability-ci/apm-server-internal   8.15.0-SNAPSHOT         d2e84aceb911   12 minutes ago   209MB

Locally

$ make package-snapshot

TBC what kind of tests should be done and how.

Related issues

#13139 will help with testing these changes in Buildkite without the need to be merged.
#12671 is also another in-progress task

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This is an automatic backport of pull request #13137 done by Mergify.