track Pictures dir via .gitkeep, declare volumes in Dockerfile

.gitignore

@@ -1,3 +1,4 @@
-Pictures/
+Pictures/*
+!Pictures/.gitkeep
 .env
 booru.db

Dockerfile

@@ -3,6 +3,6 @@ WORKDIR /app
 COPY requirements.txt .
 RUN pip install --no-cache-dir -r requirements.txt
 COPY . .
-RUN mkdir -p Pictures
+VOLUME ["/app/Pictures", "/app/booru.db"]
 EXPOSE 5000
 CMD ["python", "Slideshow.py"]