feat(train): the instance's own map tiles, drawn by Train's style (ADR 0030, D50)

Protomaps' basemap cut for Europe and served by pmtiles beside Train's server
(compose.maps.yaml), drawn by Train's own style, light and dark, which the
server serves. Only people signed in get tiles: map.source hands a key that
Caddy has the server check before each tile; nothing logs a tile.

Also fixes .dockerignore, which left apps/train/ out of the build context, so
Train's server's and BRouter's images couldn't have been built.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

deploy/.env.example

@@ -65,6 +65,12 @@ DOMAIN=http://localhost
 #MAP_SOURCE_NAME=
 #MAP_ATTRIBUTION=
 #MAP_ABOUT_URL=
+# With Train's own map (compose.maps.yaml, sidecars/README.md, Train's map; ADR 0030,
+# D50), after compose.train.yaml in COMPOSE_FILE: the secret its tiles' keys are made
+# with (openssl rand -hex 32), and the region cut, `west,south,east,north` in degrees
+# (empty: Europe's, -25,35,45,75). MAP_STYLE_URL is then not used.
+#MAP_KEY_SECRET=
+#MAP_REGION=
 
 # Postgres password, shared by the postgres and server services.
 # Generate: openssl rand -base64 32