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>

Dockerfile

@@ -0,0 +1,48 @@
+# syntax=docker/dockerfile:1
+#
+# Motionity is a static app, so the runtime image is a static file server and
+# nothing else: no Node, no shell tooling, ~8 MB of base image.
+#
+# Build:  docker build -t motionity:latest .
+# Run:    docker run --rm -p 8080:8080 motionity:latest
+#
+# WITH_FFMPEG=0 drops the 18.5 MB asm.js ffmpeg build from the image. MP4/GIF
+# export then downloads it from archive.org on first use instead of working
+# offline; everything else is unaffected.
+
+FROM node:22-alpine AS vendor
+ARG WITH_FFMPEG=1
+WORKDIR /app
+COPY scripts/vendor.mjs scripts/
+COPY src/index.html src/
+RUN if [ "$WITH_FFMPEG" = "1" ]; then \
+      node scripts/vendor.mjs; \
+    else \
+      node scripts/vendor.mjs --skip-ffmpeg; \
+    fi
+
+FROM ghcr.io/static-web-server/static-web-server:2-alpine
+
+COPY --chown=65534:65534 src/ /public/
+COPY --from=vendor --chown=65534:65534 /app/src/vendor/ /public/vendor/
+
+ENV SERVER_ROOT=/public \
+    SERVER_HOST=0.0.0.0 \
+    SERVER_PORT=8080 \
+    SERVER_COMPRESSION=true \
+    SERVER_COMPRESSION_LEVEL=default \
+    SERVER_CACHE_CONTROL_HEADERS=true \
+    SERVER_LOG_LEVEL=warn \
+    SERVER_SECURITY_HEADERS=false \
+    SERVER_HEALTH=true
+
+USER 65534:65534
+EXPOSE 8080
+
+HEALTHCHECK --interval=30s --timeout=3s --start-period=5s \
+  CMD wget -q --spider http://127.0.0.1:8080/health || exit 1
+
+# NOTE: browsers expose WebCodecs (the fast exporter) and IndexedDB (project
+# saving) only in a secure context. Reaching this container over plain http://
+# from another machine disables both. Publish it behind TLS, or keep access on
+# http://localhost.