Add .dockerignore

[Trosper3]

Summary

• There was no .dockerignore, so Dockerfile's COPY . . pulled .git/, Documentation/ (including images), build artifacts, editor metadata, and this session's .claude/ worktrees into the build context.
• Adds .dockerignore excluding .git/, .github/, bin//obj/ (matches at any depth, so this also covers redux-tests/bin, redux-tests/obj, etc. — the "test project output" the issue called out), .vs//.vscode//.idea/, .claude/, .rbs/, report.md, Documentation/, and the Dockerfile/.dockerignore themselves.
• Deliberately does NOT exclude redux-tests/ source (only its build output, via the bin//obj/ patterns): Directory.Build.targets runs dotnet format Redux.slnx --verify-no-changes before Build in Release config, and Redux.slnx includes redux-tests/redux-tests.csproj as a solution member — excluding the whole redux-tests/ directory breaks that formatting gate inside the image. Caught this by actually attempting the Docker build with the naive full-directory exclusion first; had to narrow it once dotnet format failed to open the solution workspace.

Closes #465

Test plan

• docker build from a git clone with core.autocrlf=false (an LF-normalized checkout matching what CI's Linux runners actually see — building straight from this Windows working copy hits an unrelated, pre-existing CRLF-vs-LF dotnet format mismatch that reproduces identically with or without this .dockerignore, so it isn't caused by this change) — succeeded end-to-end, build context reduced to 1.85MB
• docker run the resulting image and confirmed /swagger/index.html responds 200

[github-actions]

Redux Build System — CI Report

dotnet · commit 7969f6b

Operation	Status	Summary	Time
audit	✅	no known vulnerabilities	4.4s
format-check	✅	all files formatted	22.1s
lint	✅	0 issues	35.4s
typecheck	⏭️	covered by lint (dotnet build)	—
unit-test	❌	898 passed · coverage 70% (min 80%)	45.6s
build	✅	built local/redux:ci · 234MB	38.3s
integration-test	⏭️	no [integration] command	—
push	⏭️	not implemented	—

Overall: ❌ 4 passed · 1 failed · 3 skipped