Reduce docker build context size during elastic-agent packaging

[pchila]

Describe the enhancement:
We should reduce the amount of data we send to the docker daemon when packaging the agent.
At the time of writing we are sending ~2.5GB as docker context during the build of elastic-agent binaries. We can probably add a few entries in a .dockerignore file to exclude anything that is not needed during the docker build.
For packaging entire docker image builds we should use volume mounts to pass along big build artifacts so we should be able to reduce the context size in that case as well

Describe a specific use case for the enhancement or feature:
Running mage package
What is the definition of done?
Only necessary files are included in docker contexts when running a container during mage package

[elasticmachine]

Pinging @elastic/elastic-agent (Team:Elastic-Agent)

[elasticmachine]

Pinging @elastic/elastic-agent-control-plane (Team:Elastic-Agent-Control-Plane)