Add Docker image and multi-arch build workflow

- Add multi-stage Dockerfile (alpine, non-root, builds server+admin binaries)
- Add .dockerignore
- Add GitHub Actions workflow (also read by Gitea Actions) building and
  pushing linux/amd64 + linux/arm64 images to git.arnef.de/arnef/ebooks

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Dockerfile

@@ -0,0 +1,41 @@
+# syntax=docker/dockerfile:1
+
+FROM --platform=$BUILDPLATFORM golang:1.25-alpine AS build
+WORKDIR /src
+
+ARG TARGETOS
+ARG TARGETARCH
+
+RUN apk add --no-cache git
+
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+
+# views/pages_templ.go ist bereits generiert und im Repo eingecheckt,
+# daher kein "templ generate" im Build notwendig.
+RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH \
+    go build -trimpath -ldflags="-s -w" -o /out/server ./cmd/server
+RUN CGO_ENABLED=0 GOOS=$TARGETOS GOARCH=$TARGETARCH \
+    go build -trimpath -ldflags="-s -w" -o /out/admin ./cmd/admin
+
+FROM alpine:3.20
+RUN apk add --no-cache ca-certificates tzdata && \
+    addgroup -S ebooks && adduser -S ebooks -G ebooks
+
+WORKDIR /app
+COPY --from=build /out/server /app/server
+COPY --from=build /out/admin /app/admin
+COPY static ./static
+
+ENV ADDR=:8080
+ENV BOOKS_DIR=/data/books
+ENV USERS_DB=/data/users.db
+
+RUN mkdir -p /data/books && chown -R ebooks:ebooks /data /app
+VOLUME ["/data"]
+USER ebooks
+
+EXPOSE 8080
+ENTRYPOINT ["/app/server"]