perf(docker): add .dockerignore so the build context stops shipping .git

[Harshithraj24]

Small cleanup, not the big win — worth being clear about that up front.

We have no .dockerignore, so COPY . . ships the whole build context. The large part of that was .git, but the GitHub Actions workflow already checks out with fetch-depth: 1, so once transpiler builds move there the history is gone anyway. 3.0.1011 shows it: that build's COPY . . layer is 39 MB against 515 MB on the Jenkins-built 3.0.211.

What fetch-depth: 1 doesn't remove is everything else we don't need at runtime — docs/ is 80 MB in the tree, plus tests/, posts/, benchmarks/ and the README images. That's the remaining 39 MB layer, where the runtime tree is under 1 MB.

So this is worth roughly 38 MB per build, not the 500 MB you'd get from the checkout depth. It's free and it belongs in the repo rather than in CI config, but it isn't urgent.

Kept requirements.txt and sqlglotrs/ in the context since the build needs them. Also checked that dropping .git doesn't break versioning — nothing here runs pip install ., and sqlglotrs takes its version from Cargo.toml.

Not built locally (no Docker on my machine), so worth a glance at the layer sizes on the first CI build.