bump to python3 and sigal 2

Dockerfile

@@ -1,17 +1,12 @@
 FROM nginx:stable
 
-VOLUME /pictures
-
 WORKDIR /opt
 
 RUN apt-get update && apt-get install -y \
-    python-dev \
-    python-pip \
-    zlib1g-dev \
-    libjpeg-dev \
-    libfreetype6-dev \
- && rm -rf /var/lib/apt/lists/*
-RUN pip install sigal
+    python3-pip \
+    && rm -rf /var/lib/apt/lists/*
+
+RUN pip3 install sigal
 
 COPY run.sh sigal.conf.py auth.conf auth.htpasswd ./