chore: cleanup Dockerfile for custom Kpler usage

Dockerfile

@@ -1,51 +1,47 @@
-FROM python:3.8-slim-buster
-# the args must be onbuild to allow overwriting it in derived images
-# https://github.com/moby/moby/issues/26533#issuecomment-246966836
-ONBUILD ARG PIP_INDEX_URL
-ONBUILD ARG PIP_TRUSTED_HOST
-ONBUILD ARG APT_PROXY
-ONBUILD ENV PIP_TRUSTED_HOST=$PIP_TRUSTED_HOST PIP_INDEX_URL=$PIP_INDEX_URL
-ONBUILD RUN if [ -n "$APT_PROXY" ]; then \
-    echo "Acquire::http::Proxy \"$APT_PROXY\";" >/etc/apt/apt.conf.d/proxy; fi
+FROM python:3.6-slim-buster
 
-# TERM needs to be set here for exec environments
+# TERM needs to be set here for scrapinghub exec environments
 # PIP_TIMEOUT so installation doesn't hang forever
 ENV TERM=xterm \
     PIP_TIMEOUT=180 \
     SHUB_ENFORCE_PIP_CHECK=1
 
-RUN apt-get update -qq && \
-    apt-get install -qy \
-        netbase ca-certificates apt-transport-https \
-        build-essential locales \
-        default-libmysqlclient-dev \
-        imagemagick \
-        libbz2-dev \
-        libdb-dev \
-        libevent-dev \
-        libffi-dev \
-        libjpeg-dev \
-        liblzma-dev \
-        libpcre3-dev \
-        libpng-dev \
-        libpq-dev \
-        libsqlite3-dev \
-        libssl-dev \
-        libxml2-dev \
-        libxslt1-dev \
-        libz-dev \
-        unixodbc unixodbc-dev \
-        telnet vim htop iputils-ping curl wget lsof git sudo \
-        ghostscript && \
-    rm -rf /var/lib/apt/lists
+RUN apt-get update -qq
+RUN apt-get install -qy \
+    netbase ca-certificates apt-transport-https \
+    build-essential locales \
+    default-libmysqlclient-dev \
+    imagemagick \
+    libbz2-dev \
+    libdb-dev \
+    libevent-dev \
+    libffi-dev \
+    libjpeg-dev \
+    liblzma-dev \
+    libpcre3-dev \
+    libpng-dev \
+    libpq-dev \
+    libsqlite3-dev \
+    libssl-dev \
+    libxml2-dev \
+    libxslt1-dev \
+    libz-dev \
+    unixodbc unixodbc-dev \
+    telnet vim htop iputils-ping curl wget lsof git sudo \
+    ghostscript
 
-# adding custom locales to provide backward support with scrapy cloud 1.0
+# space optimization to remove apt cache
+RUN rm -rf /var/lib/apt/lists
+
+# ad custom locales to provide backward support with scrapy cloud 1.0
 COPY locales /etc/locale.gen
 RUN locale-gen
 
+# requirements for scrapy stack as deployed on Scrapinghub
 COPY requirements.txt /stack-requirements.txt
 RUN pip install --no-cache-dir -r stack-requirements.txt
 
+# scrapinghub builtin addons
 RUN mkdir /app
 COPY addons_eggs /app/addons_eggs
 RUN chown nobody:nogroup -R /app/addons_eggs