chore: remove in-tree-build flag as it is the default now (fixes #19)

Author: YoRyan

Dockerfile

@@ -2,7 +2,7 @@
 FROM python:3 AS builder
 WORKDIR /code
 COPY . .
-RUN pip install --user --no-cache-dir --use-feature=in-tree-build --no-warn-script-location .
+RUN pip install --user --no-cache-dir --no-warn-script-location .
 
 # second stage
 FROM python:3-slim

Dockerfile.buster

@@ -2,7 +2,7 @@
 FROM python:3.9-buster AS builder
 WORKDIR /code
 COPY . .
-RUN pip install --user --no-cache-dir --use-feature=in-tree-build --no-warn-script-location .
+RUN pip install --user --no-cache-dir --no-warn-script-location .
 
 # second stage
 FROM python:3.9-slim-buster