add: `dockerignore`; change: df layers handling

.dockerignore

@@ -0,0 +1,6 @@
+.git
+.gitignore
+config.ini
+README.md
+TODO
+requirements.txt

Dockerfile

@@ -1,6 +1,11 @@
 FROM python:3.10-alpine
-COPY . /app
+
 WORKDIR /app
-RUN pip install -r /app/requirements.txt
+
+COPY requirements.txt /app/requirements.txt
+
+RUN pip install -r /app/requirements.txt && rm /app/requirements.txt
+
+COPY . /app
 
 CMD ["python3", "-m", "mastoposter", "/config.ini"]