refactor: use .python-version for docker build

scripts/docker-run

@@ -1,6 +1,7 @@
 #!/bin/sh -eu
 
 : "${DOCKER_BIN:=docker}"
+: "${UNWIND_PORT:=8097}"
 
 cd "$RUN_DIR"
 
@@ -8,11 +9,18 @@ cd "$RUN_DIR"
 
 version=$(cat build/version)
 
+localhost=127.0.0.1
+localport=8000
+netloc="$localhost:$localport"
+
+echo >&2 "Unwind will be available on http://$netloc/
+"
+
 $DOCKER_BIN run \
     --init \
     -it --rm \
     --read-only \
     --memory '500m' \
-    --publish 127.0.0.1:8000:8000 \
+    --publish "$netloc":"$UNWIND_PORT" \
     --volume "$RUN_DIR"/data:/data \
     "code.dumpr.org/ducklet/unwind":"$version"