Bundle go-pmtiles into the Docker image

The "Regenerate for current view" build was failing in production with
"could not run 'pmtiles'" — go-pmtiles was never actually installed
anywhere, just documented as something a developer runs by hand
locally (docs/development.adoc). Installs the static Linux binary from
go-pmtiles' own GitHub releases, architecture-matched via dpkg rather
than hardcoded amd64.

Verified the exact download/extract/exec sequence end-to-end outside
Docker (no daemon available in this sandbox to run a real build).

docs/third-party-notices.adoc

@@ -82,6 +82,24 @@ The online map style (not vendored, fetched at runtime when the
 https://tiles.openfreemap.org/styles/liberty[OpenFreeMap "liberty"],
 built from https://www.openstreetmap.org[OpenStreetMap] data.
 
+Not a frontend asset like the above, but also baked into this app's own
+Docker image (not a separate container the way kiwix-serve/gpsd are —
+see `docs/deployment.adoc`/`docs/development.adoc`): the `pmtiles` CLI
+(`go-pmtiles`), installed as a static binary in the `Dockerfile` for
+`lib/maptool/pmtiles_builder.rb`'s in-app offline.pmtiles/terrain.pmtiles
+regeneration. BSD-3-Clause, same as pmtiles.js/MapLibre GL above.
+
+[cols="1,1,2"]
+|===
+| Tool | Version | Source
+
+| go-pmtiles (`pmtiles` CLI)
+| 1.31.2
+| https://github.com/protomaps/go-pmtiles
+|===
+
 To update a vendored library: download its package (e.g.
 `npm pack <name>@<version>`), replace the corresponding files under
-`public/vendor/<name>/`, and update the version in this file.
+`public/vendor/<name>/`, and update the version in this file. To
+update go-pmtiles, bump `PMTILES_VERSION` in the `Dockerfile` and the
+version here.