Build DuckDB with the ui extension and import sources via a build context

- Build the ui extension (pinned duckdb-ui commit for DuckDB 1.5.6) into
  DuckDB on debian13 and rocky9 so the shell supports -ui.
- Add fetch-sources.sh, which downloads source archives with aria2c into the
  git-ignored sources/ repository.
- debian13: replace per-dependency downloads with
  RUN --mount=type=bind,from=sources, passed by build.sh through
  --build-context, so archives are not stored in image layers.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

debian13/build.sh

@@ -4,11 +4,15 @@ set -eu
 
 IMAGE_NAME="${IMAGE_NAME:-debian13-source-stack}"
 SCRIPT_DIR=$(CDPATH= cd -- "$(dirname -- "$0")" && pwd)
+SOURCES_DIR="${SOURCES_DIR:-$SCRIPT_DIR/../sources}"
+FETCH_SCRIPT="$SCRIPT_DIR/../fetch-sources.sh"
 
 if ! command -v podman >/dev/null 2>&1; then
     echo "Error: Podman is required." >&2
     exit 1
 fi
 
+SOURCES_DIR="$SOURCES_DIR" "$FETCH_SCRIPT"
+
 echo "Building image $IMAGE_NAME with podman ..."
-exec podman build --tag "$IMAGE_NAME" "$SCRIPT_DIR"
+exec podman build --build-context sources="$SOURCES_DIR" --tag "$IMAGE_NAME" "$SCRIPT_DIR"