Work on #152

.devcontainer/check-image.sh

@@ -0,0 +1,24 @@
+#!/usr/bin/env bash
+# initializeCommand for the dev container: runs on the *host* before the
+# container is created. The image is built locally (it embeds proprietary
+# SDKs, so it is not on a registry); without this check a missing image
+# surfaces as a confusing "pull access denied" from Docker.
+set -euo pipefail
+
+image="${1:?usage: check-image.sh IMAGE:TAG}"
+
+if ! docker image inspect "${image}" >/dev/null 2>&1; then
+  cat >&2 <<EOF
+
+ERROR: dev-container image '${image}' is not built yet.
+
+Build it first (from the repo root; stage 1 takes a while, see docker/README.md):
+
+    docker/gdal/build.sh      # stage 1: GDAL + ECW + MrSID
+    docker/python/build.sh    # stage 2: uv venv with rioxarray/rasterio/...
+
+then reopen the folder in the container.
+
+EOF
+  exit 1
+fi