feat(PROJ-27): Docker-Support + install.sh v2.0 (native + Docker-Modus)

Adds multi-stage Dockerfiles (Go+Xapian CGO, Next.js standalone),
docker-compose.yml, webhook-basierter Deploy-Flow und erweitertes
install.sh mit interaktiver Modus-Auswahl.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Dockerfile

@@ -0,0 +1,37 @@
+# ── Stage 1: Go Backend Build (CGO + Xapian) ─────────────────────────────────
+FROM golang:1.24-bookworm AS go-builder
+
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libxapian-dev pkg-config build-essential \
+    && rm -rf /var/lib/apt/lists/*
+
+WORKDIR /build
+COPY go.mod go.sum ./
+RUN go mod download
+
+COPY . .
+RUN CGO_ENABLED=1 go build -tags xapian -buildvcs=false \
+    -ldflags="-s -w" \
+    -o /out/archivmail ./cmd/archivmail/
+
+# ── Stage 2: Runtime Image ────────────────────────────────────────────────────
+FROM debian:bookworm-slim
+
+# Xapian runtime library only (no dev headers)
+RUN apt-get update && apt-get install -y --no-install-recommends \
+    libxapian30 ca-certificates \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN useradd --system --shell /bin/false --uid 999 --create-home archivmail
+
+COPY --from=go-builder /out/archivmail /usr/local/bin/archivmail
+
+# Volumes: config + data werden von außen gemountet
+RUN mkdir -p /var/archivmail/store /var/archivmail/astore /var/archivmail/xapian \
+    && chown -R archivmail:archivmail /var/archivmail
+
+USER archivmail
+EXPOSE 8080 2525 1143
+
+ENTRYPOINT ["/usr/local/bin/archivmail"]
+CMD ["-config", "/etc/archivmail/config.yml"]