add dockerfile for backend && update docker-compose.yml

backend/Dockerfile

@@ -0,0 +1,34 @@
+# Stage 1: Builder
+FROM golang:1.25-alpine AS builder
+
+WORKDIR /app
+
+# Install git and build dependencies
+RUN apk add --no-cache git
+
+# Copy go mod files
+COPY go.mod go.sum ./
+RUN go mod download
+
+# Copy source code
+COPY . .
+
+# Build the application
+RUN CGO_ENABLED=0 GOOS=linux go build -o api ./cmd/api
+
+# Stage 2: Production
+FROM alpine:latest AS production
+
+RUN apk --no-cache add ca-certificates
+
+WORKDIR /app
+
+# Copy binary from builder
+COPY --from=builder /app/api .
+
+# Copy migrations
+COPY internal/db/migrations ./migrations
+
+EXPOSE 8080
+
+CMD ["./api"]