Add multi-arch build support and detailed antenna guide

Multi-arch Docker builds:
- build-multiarch.sh: Cross-compile amd64+arm64 on x64 and push to
  registry, so RPi5 can docker pull instead of building natively
- docker-compose.yml: Add INTERCEPT_IMAGE env var to support pulling
  pre-built images from a registry instead of local build
- README.md: Docker build section rewritten with multi-arch workflow,
  registry pull instructions, and build script options

Weather satellite antenna guide (sidebar panel):
- V-Dipole: ASCII diagram, 53.4cm element length, 120 degree angle,
  materials, orientation, connection instructions
- Turnstile/Crossed Dipole: phasing coax length (37cm RG-58),
  reflector distance (52cm below), RHCP explanation
- QFH Quadrifilar Helix: design overview, materials, height (46cm),
  hemispherical gain pattern
- Placement & LNA: outdoor requirements, coax loss figures,
  LNA mounting position, Nooelec SAWbird+ recommendation, Bias-T
- Quick reference table: wavelength, quarter-wave, elevation,
  duration, polarization, APT/LRPT bandwidth

Also added Weather Satellites and ISS SSTV to README features list,
SatDump to acknowledgments.

https://claude.ai/code/session_01FjLTkyELaqh27U1wEXngFQ

build-multiarch.sh

@@ -0,0 +1,139 @@
+#!/bin/bash
+# INTERCEPT - Multi-architecture Docker image builder
+#
+# Builds for both linux/amd64 and linux/arm64 using Docker buildx.
+# Run this on your x64 machine to cross-compile the arm64 image
+# instead of building natively on the RPi5.
+#
+# Prerequisites (one-time setup):
+#   docker run --privileged --rm tonistiigi/binfmt --install all
+#   docker buildx create --name intercept-builder --use --bootstrap
+#
+# Usage:
+#   ./build-multiarch.sh                    # Build both platforms, load locally
+#   ./build-multiarch.sh --push             # Build and push to registry
+#   ./build-multiarch.sh --arm64-only       # Build arm64 only (for RPi)
+#   REGISTRY=ghcr.io/user ./build-multiarch.sh --push
+#
+# Environment variables:
+#   REGISTRY    - Container registry (default: docker.io/library)
+#   IMAGE_NAME  - Image name (default: intercept)
+#   IMAGE_TAG   - Image tag (default: latest)
+
+set -euo pipefail
+
+# Configuration
+REGISTRY="${REGISTRY:-}"
+IMAGE_NAME="${IMAGE_NAME:-intercept}"
+IMAGE_TAG="${IMAGE_TAG:-latest}"
+BUILDER_NAME="intercept-builder"
+PLATFORMS="linux/amd64,linux/arm64"
+
+# Parse arguments
+PUSH=false
+LOAD=false
+ARM64_ONLY=false
+
+for arg in "$@"; do
+    case $arg in
+        --push)     PUSH=true ;;
+        --load)     LOAD=true ;;
+        --arm64-only)
+            ARM64_ONLY=true
+            PLATFORMS="linux/arm64"
+            ;;
+        --amd64-only)
+            PLATFORMS="linux/amd64"
+            ;;
+        --help|-h)
+            echo "Usage: $0 [--push] [--load] [--arm64-only] [--amd64-only]"
+            echo ""
+            echo "Options:"
+            echo "  --push        Push to container registry"
+            echo "  --load        Load into local Docker (single platform only)"
+            echo "  --arm64-only  Build arm64 only (for RPi5 deployment)"
+            echo "  --amd64-only  Build amd64 only"
+            echo ""
+            echo "Environment variables:"
+            echo "  REGISTRY    Container registry (e.g. ghcr.io/username)"
+            echo "  IMAGE_NAME  Image name (default: intercept)"
+            echo "  IMAGE_TAG   Image tag (default: latest)"
+            echo ""
+            echo "Examples:"
+            echo "  $0 --push                                    # Build both, push"
+            echo "  REGISTRY=ghcr.io/myuser $0 --push            # Push to GHCR"
+            echo "  $0 --arm64-only --load                       # Build arm64, load locally"
+            echo "  $0 --arm64-only --push && ssh rpi docker pull # Build + deploy to RPi"
+            exit 0
+            ;;
+        *)
+            echo "Unknown option: $arg"
+            exit 1
+            ;;
+    esac
+done
+
+# Build full image reference
+if [ -n "$REGISTRY" ]; then
+    FULL_IMAGE="${REGISTRY}/${IMAGE_NAME}:${IMAGE_TAG}"
+else
+    FULL_IMAGE="${IMAGE_NAME}:${IMAGE_TAG}"
+fi
+
+echo "============================================"
+echo "  INTERCEPT Multi-Architecture Builder"
+echo "============================================"
+echo "  Image:     ${FULL_IMAGE}"
+echo "  Platforms: ${PLATFORMS}"
+echo "  Push:      ${PUSH}"
+echo "============================================"
+echo ""
+
+# Check if buildx builder exists, create if not
+if ! docker buildx inspect "$BUILDER_NAME" >/dev/null 2>&1; then
+    echo "Creating buildx builder: ${BUILDER_NAME}"
+    docker buildx create --name "$BUILDER_NAME" --use --bootstrap
+
+    # Check for QEMU support
+    if ! docker run --rm --privileged tonistiigi/binfmt --install all >/dev/null 2>&1; then
+        echo "WARNING: QEMU binfmt setup may have failed."
+        echo "Run: docker run --privileged --rm tonistiigi/binfmt --install all"
+    fi
+else
+    docker buildx use "$BUILDER_NAME"
+fi
+
+# Build command
+BUILD_CMD="docker buildx build --platform ${PLATFORMS} --tag ${FULL_IMAGE}"
+
+if [ "$PUSH" = true ]; then
+    BUILD_CMD="${BUILD_CMD} --push"
+    echo "Will push to: ${FULL_IMAGE}"
+elif [ "$LOAD" = true ]; then
+    # --load only works with single platform
+    if echo "$PLATFORMS" | grep -q ","; then
+        echo "ERROR: --load only works with a single platform."
+        echo "Use --arm64-only or --amd64-only with --load."
+        exit 1
+    fi
+    BUILD_CMD="${BUILD_CMD} --load"
+    echo "Will load into local Docker"
+fi
+
+echo ""
+echo "Building..."
+echo "Command: ${BUILD_CMD} ."
+echo ""
+
+$BUILD_CMD .
+
+echo ""
+echo "============================================"
+echo "  Build complete!"
+if [ "$PUSH" = true ]; then
+    echo "  Image pushed to: ${FULL_IMAGE}"
+    echo ""
+    echo "  Pull on RPi5:"
+    echo "    docker pull ${FULL_IMAGE}"
+fi
+echo "============================================"