feat: add Docker deployment support

Add Dockerfile, docker-compose.yml, .dockerignore, .env.example, Makefile docker targets, and README/CHANGELOG docs.

Dockerfile

@@ -0,0 +1,19 @@
+# syntax=docker/dockerfile:1
+
+FROM golang:1.25-alpine AS build
+WORKDIR /src
+COPY go.mod go.sum ./
+RUN go mod download
+COPY . .
+RUN CGO_ENABLED=0 GOOS=linux go build -ldflags="-s -w" -o /out/yDropbox ./cmd/ydropbox
+
+FROM alpine:3.20
+RUN adduser -D -u 10001 -s /sbin/nologin ydropbox \
+    && apk add --no-cache ca-certificates tzdata
+WORKDIR /app
+COPY --from=build /out/yDropbox /app/yDropbox
+USER ydropbox
+VOLUME /app/workspace
+EXPOSE 8999
+ENTRYPOINT ["/app/yDropbox"]
+CMD ["--addr", "0.0.0.0:8999", "--workspace", "/app/workspace"]