Add Docker setup, .env.example, updated .gitignore/CHANGELOG and ADRs 004-007 + arc42 template

backend/chat/Dockerfile

@@ -0,0 +1,17 @@
+FROM golang:1.23-alpine AS builder
+
+WORKDIR /app
+COPY src/ ./src/
+COPY go.mod go.sum ./
+
+RUN go mod download
+RUN CGO_ENABLED=0 GOOS=linux go build -o /chat-server ./src/main.go
+
+FROM alpine:3.20
+RUN apk --no-cache add ca-certificates
+WORKDIR /app
+COPY --from=builder /chat-server .
+COPY config/ ./config/
+
+EXPOSE 3000
+CMD ["./chat-server"]