Add Docker support and GitHub Actions workflow

backend/Dockerfile

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+FROM golang:1.21-alpine AS builder
+
+WORKDIR /app
+
+RUN apk add --no-cache git
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+RUN CGO_ENABLED=0 GOOS=linux go build -o /app/server ./cmd/server
+
+FROM alpine:latest
+
+WORKDIR /app
+
+RUN apk add --no-cache ca-certificates tzdata
+
+COPY --from=builder /app/server .
+COPY --from=builder /app/uploads ./uploads
+
+EXPOSE 8080
+
+CMD ["./server"]