Add everything needed to have a dockerfile for agent testing

Part of https://github.com/elastic/search-team/issues/8211

This change provides a way to run docker image of agent with connectors component installed.

It's used purely for dev/testing purposes, should not be used for anything else.

When we finish the work with Agent, this docker image might change significantly or will be deleted.

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[x] this PR links to all relevant github issues that it fixes or partially addresses
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[x] this PR has a thorough description
[x] Covered the changes with automated tests
[x] Tested the changes locally
[x] Added a label for each target release version (example: v7.13.2, v7.14.0, v8.0.0)