Add Dockerfile, nginx config, and Gitea Actions CI workflow

Multi-stage Docker build produces an Nginx container serving the
static SPA. The CI workflow runs pnpm check on every push and
builds/pushes a Docker image on semver tags.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,19 @@
+FROM node:22-slim AS build
+ENV PNPM_HOME="/pnpm"
+ENV PATH="$PNPM_HOME:$PATH"
+RUN corepack enable
+
+WORKDIR /app
+COPY package.json pnpm-lock.yaml pnpm-workspace.yaml ./
+COPY packages/domain/package.json packages/domain/
+COPY packages/application/package.json packages/application/
+COPY apps/web/package.json apps/web/
+RUN pnpm install --frozen-lockfile
+
+COPY . .
+RUN pnpm --filter web build
+
+FROM nginx:alpine
+COPY --from=build /app/apps/web/dist /usr/share/nginx/html
+COPY nginx.conf /etc/nginx/conf.d/default.conf
+EXPOSE 80