Move environment variables to middleware. Closes #972.

Dockerfile

@@ -2,12 +2,11 @@
 FROM node:12.22-alpine AS build
 ARG BASE_PATH
 ARG DATABASE_TYPE
-ARG TRACKER_SCRIPT_NAME
 
 ENV BASE_PATH=$BASE_PATH
 ENV DATABASE_URL "postgresql://umami:umami@db:5432/umami"
 ENV DATABASE_TYPE=$DATABASE_TYPE
-ENV TRACKER_SCRIPT_NAME=$TRACKER_SCRIPT_NAME
+
 WORKDIR /build
 
 RUN yarn config set --home enableTelemetry 0