Prevent slow-to-start containers from trying to run in the same slot

[dkouba-atym]

Quick fix for multi-container startup race condition when a container is slow to start. Container slots remain free until container startup is complete, meaning multiple containers can identify the same slot as open to occupy. This fix creates a reserved state that is immediately applied to an open container slot once identified.

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Also, please list any issue that this PR is intended to fix.

• Fixes issue #

Type of change

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• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Code cleanup/refactoring
• This change requires a documentation update
• CI system update
• Test Coverage update

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration including the following required details.

Test Configuration (required):

• Host OS type, version, and arch: Ubuntu 20.04 (arm64)
• Developer Board make & model: ST Nucleo h723zg
• Board Tools Installed: STM32CubeProgrammer)

Checklist:

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes