build: add container image and ignore rules

Dockerfile builds a static-web-server image serving src/ with the third-party
assets vendored in a first stage, so the runtime layer carries no Node and no
shell tooling. WITH_FFMPEG=0 drops the 18.5 MB asm.js encoder for a smaller
image, at the cost of fetching it at export time.

.gitignore covers dist/, which the release scripts write on every run: without
it a fresh clone reports the build output as untracked and publish.ps1 warns
that the worktree is dirty on every build.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

.dockerignore

@@ -0,0 +1,13 @@
+.git
+.github
+.playwright-mcp
+node_modules
+dist
+build
+electron
+test
+deploy
+*.md
+
+# Always re-downloaded inside the build so the image is reproducible.
+src/vendor