feat: add Docker setup with multi-stage build and docker-compose

- Add Dockerfile with golang:1.25-alpine build stage + alpine runtime stage
- Add docker-compose.yml with naviwatcher service and data volume
- Add .dockerignore for build context cleanup
- Add .gitignore for build artifacts and data directory
- Update plan: mark Task 7 checkboxes as complete

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

Dockerfile

@@ -0,0 +1,29 @@
+# Build stage
+FROM golang:1.25-alpine AS builder
+
+RUN apk add --no-cache gcc musl-dev
+
+WORKDIR /app
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN CGO_ENABLED=1 GOOS=linux go build -o naviwatcher ./cmd/naviwatcher
+
+# Runtime stage
+FROM alpine:latest
+
+RUN apk add --no-cache ca-certificates sqlite-libs
+
+WORKDIR /app
+
+COPY --from=builder /app/naviwatcher .
+
+EXPOSE 8080
+
+VOLUME ["/app/data"]
+
+ENTRYPOINT ["./naviwatcher"]
+CMD ["-config=/app/data/config.yaml"]