feat: add Dockerfile and CI workflow

Add multi-stage Dockerfile producing a minimal distroless image and
Gitea Actions CI workflow for automated testing and image publishing.

- Dockerfile: multi-stage build (golang:1.22-alpine -> distroless/static)
  with stripped binary (~15-20MB image), runs as nonroot user
- .dockerignore: excludes .git, docs, nix files from build context
- .gitea/workflows/build.yaml: CI pipeline that runs tests, builds
  Docker image, and pushes to Gitea registry with timestamp+SHA tags
  for Flux image automation

Closes leeworks-agents/gitea-mobile#7

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,16 @@
+# Stage 1: Build
+FROM golang:1.22-alpine AS builder
+WORKDIR /app
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /gitea-mobile ./cmd/server
+
+# Stage 2: Runtime
+FROM gcr.io/distroless/static:nonroot
+COPY --from=builder /gitea-mobile /gitea-mobile
+COPY static/ /static/
+COPY internal/templates/ /templates/
+EXPOSE 8080
+USER nonroot:nonroot
+ENTRYPOINT ["/gitea-mobile"]