Add build docker image workflow

docker/build.dockerfile

@@ -0,0 +1,21 @@
+FROM rust:bullseye AS builder
+
+WORKDIR /app
+
+COPY . .
+
+RUN cargo build --release --bin ionos-ddns
+
+
+FROM debian:bullseye-slim
+
+RUN apt-get update \
+    && apt-get install -y openssl ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN update-ca-certificates
+
+WORKDIR /app
+
+COPY --from=builder /app/target/release/ionos-ddns /usr/local/bin
+ENTRYPOINT ["/usr/local/bin/ionos-ddns"]