Fixed Docker build error

Dockerfile

@@ -3,11 +3,13 @@ FROM ubuntu:18.04
 WORKDIR /usr/src/app
 RUN chmod 777 /usr/src/app
 RUN apt-get -qq update
-RUN apt-get -qq install -y aria2 python3 python3-pip \
+RUN apt-get -qq install -y aria2 git python3 python3-pip \
     locales python3-lxml \
     curl pv jq ffmpeg
 COPY requirements.txt .
-RUN pip3 install --no-cache-dir -r requirements.txt
+RUN pip3 install --no-cache-dir -r requirements.txt && \
+    apt-get -qq purge git
+
 RUN locale-gen en_US.UTF-8
 ENV LANG en_US.UTF-8
 ENV LANGUAGE en_US:en