Publish `latest` tag for Docker image

[felixbarny]

For the k8s webhook, we want to provide a default configuration that attaches the latest Java agent.

Currently, our Docker image doesn't publish the latest tag.

We need to adjust how we publish docker images on release (https://github.com/elastic/apm-agent-java/blob/main/scripts/jenkins/push_docker.sh). One thing to watch out for is that bug fix releases on a previous minor should not be tagged latest. Example:

• release 1.0.0 latest=1.0.0
• release 1.1.0 latest=1.1.0
• release 1.0.1 latest=1.0.0
• release 1.1.1 latest=1.1.1

• Make sure that the default is latest in the K8s attachment web-hook.

[felixbarny]

In addition to the latest tag, we might also want to create a 1.x tag.

[felixbarny]

After releasing the next version of the agent, we need to update the webhook config: https://github.com/elastic/apm-mutating-webhook/blob/973a447b44519d189a466d6112769c8243b89777/apm-agent-auto-attach/values.yaml#L29

[SylvainJuge]

After releasing the next version of the agent, we need to update the webhook config: https://github.com/elastic/apm-mutating-webhook/blob/973a447b44519d189a466d6112769c8243b89777/apm-agent-auto-attach/values.yaml#L29

Is that all is left to complete elastic/apm-k8s-attacher#17 ?