feat: add dockerShmSize option to raise Docker's /dev/shm limit - #81

Brings the fix from unity-test-runner#308 (mob-sakai) into the CLI, since build/test logic now lives here rather than in the action repos directly.

What

Unity 6.6 beta / 6.7 alpha editors have been reported to fail against Docker's 64MB default /dev/shm:

Error: Requested 1073741824 bytes, but only 67108864 bytes available
Error: Insufficient shared memory available - if using Docker, please run the container with --shm-size=1025M

Adds a dockerShmSize option (empty/unset by default — matches Docker's own default and doesn't change behavior for editors that don't need it), following the exact pattern of the existing dockerCpuLimit/dockerMemoryLimit options. Wired into both the Linux and Windows docker run command builders in Docker.

Why not port the original PR's unconditional --shm-size=1025m

The original fix hardcoded --shm-size=1025m on every run. Making it an opt-in flag (--docker-shm-size 1025m) instead avoids silently changing resource allocation for every user on every Unity version, while still giving people hitting this specific error (older/beta editors) a documented way to fix it — same shape as the other Docker resource options already here.

Testing

• New tests in docker.test.ts: --shm-size appears on both Linux and Windows commands when dockerShmSize is set, and is omitted entirely when unset.
• bun run test — 148 pass, 3 skip, 0 fail.

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