Document design choices and standardize Dockerfile

Dockerfile

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+FROM golang:1.22-bookworm
+
+# Fyne links against native desktop libraries, so the container must include a C
+# compiler plus OpenGL/X11 headers. --no-install-recommends keeps the image from
+# pulling in unrelated desktop packages that are not needed for compilation.
+RUN apt-get update && \
+    apt-get install -y --no-install-recommends \
+        ca-certificates \
+        gcc \
+        libgl1-mesa-dev \
+        xorg-dev && \
+    rm -rf /var/lib/apt/lists/*
+
+WORKDIR /src
+
+# Copy module files first so Docker can cache downloaded dependencies while the
+# application source changes. This makes repeated local builds much faster.
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# CGO is required by Fyne. The first Linux package target is linux/amd64; other
+# architectures can be added later as separate, explicit build targets.
+ENV CGO_ENABLED=1
+ENV GOOS=linux
+ENV GOARCH=amd64
+
+# -trimpath removes host paths from the binary, and -s -w strips symbol/debug
+# tables to keep the produced desktop executable smaller.
+RUN go build -trimpath -ldflags "-s -w" -o /out/pysentry ./cmd/pysentry