feat(ci/cd): improve Dockerfile

Dockerfile

@@ -8,10 +8,12 @@ ENV WEB_SERVER=$WEB_SERVER
 
 WORKDIR /app
 
-COPY . .
+COPY requirements.txt .
 
 RUN pip install --no-cache-dir -r requirements.txt
 
+COPY . .
+
 # AFAIK, there's no way to expose
 # a port condtionally. The container
 # has to expose this, even if it