Add gnomock.WithShmSize option

[bayandin]

This PR adds gnomock.WithShmSize option, allowing to configure a container’s shared memory size (just like Docker’s --shm-size flag).

[Copilot]

Pull request overview

This PR adds support for configuring Docker container shared memory size through a new gnomock.WithShmSize option, mirroring Docker's --shm-size flag functionality. This is particularly useful for applications like PostgreSQL that may require more than the default 64MB of shared memory.

Key Changes:

• Introduces WithShmSize option function for setting container shared memory size in bytes
• Adds ShmSize field to the Options struct with appropriate JSON serialization
• Implements conditional application of the shared memory size in container creation (only when > 0)

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
options.go	Adds WithShmSize option function and ShmSize field to Options struct with documentation
docker.go	Implements the shared memory size configuration by conditionally setting hostConfig.ShmSize during container creation
gnomock_test.go	Adds comprehensive test that verifies the shared memory size is correctly applied by inspecting the created container

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