Parameterize compose per instance and add Spotify credential refresh script

Every clone can now override COMPOSE_PROJECT_NAME, CONTAINER_NAME, MPD_PORT,
HTTP_PORT, MOPIDY_CONFIG and MUSIC_DIR via .env to run an isolated Mopidy
instance (see .env.example). scripts/refresh-spotify-creds.py regenerates the
librespot credentials blob for whichever instance the local .env points at.

.env.example

@@ -0,0 +1,20 @@
+# Copy to .env and adjust per instance. Give every clone a unique
+# COMPOSE_PROJECT_NAME and a unique set of ports.
+
+# Namespaces the named volumes (mopidy-data, mopidy-cache).
+COMPOSE_PROJECT_NAME=mopidy
+
+# Container name shown in `docker ps`.
+CONTAINER_NAME=mopidy
+
+# Host ports for MPD/JSON-RPC and the HTTP API.
+MPD_PORT=6600
+HTTP_PORT=6680
+
+# This instance's config file and music library.
+MOPIDY_CONFIG=./config/mopidy.conf
+MUSIC_DIR=./music
+
+# Override the pip packages installed at startup (defaults to the
+# mopidy-webhooks bundle plus the web clients).
+#EXTRA_PIP_PACKAGES=