feat(docker): Preseed cities database and fix multi-arch builds

Bake the GeoNames cities database into the image at build time instead
of relying on host-mounted sqlite files, and fix the Go build stage to
cross-compile for the requested target platform instead of hardcoding
amd64 (was silently broken for arm64 builds).

docker-compose.yml

@@ -8,9 +8,4 @@ services:
       - "8000:8000"
     environment:
       PORT: "8000"
-      DB_PATH: "/data/prayertimes.sqlite3"
-    volumes:
-      - ./prayertimes.sqlite3:/data/prayertimes.sqlite3
-      - ./prayertimes.sqlite3-wal:/data/prayertimes.sqlite3-wal
-      - ./prayertimes.sqlite3-shm:/data/prayertimes.sqlite3-shm
     restart: unless-stopped