[Agent] Replace dockerapi container with Redis-based control bus

data/Dockerfiles/agent/Dockerfile

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+# Builder image for mailcow-agent. Each service Dockerfile pulls the static
+# binary from here via:
+#
+#   COPY --from=ghcr.io/mailcow/agent:VERSION /out/mailcow-agent /usr/local/bin/mailcow-agent
+#
+# For local development: build this image first.
+#
+#   docker build -t ghcr.io/mailcow/agent:dev data/Dockerfiles/agent/
+#
+# CI publishes a versioned tag and the service Dockerfiles pin against it via
+# ARG AGENT_IMAGE.
+
+FROM golang:1.22-alpine AS build
+
+ENV CGO_ENABLED=0 \
+    GOOS=linux
+
+WORKDIR /src
+
+COPY go.mod go.sum* ./
+RUN go mod download
+
+COPY . .
+
+RUN mkdir -p /out \
+ && go build -trimpath -ldflags="-s -w" \
+    -o /out/mailcow-agent ./cmd/mailcow-agent \
+ && cp mailcow-agent-cli /out/mailcow-agent-cli \
+ && chmod +x /out/mailcow-agent-cli
+
+# Final stage: tiny image whose only purpose is to be a COPY --from source.
+FROM scratch
+COPY --from=build /out/mailcow-agent /out/mailcow-agent
+COPY --from=build /out/mailcow-agent-cli /out/mailcow-agent-cli