Dockerfile: Update for coala-bears master

The Dockerfile automatically started using Python 3.7
when Alpine starting using that has its default python
image.

coala-bears master does not support Python 3.7 yet.

Also a new bear depends on libbrotli, which requires
stdc++.

Related to coala#496
Related to coala/meta#123

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3-alpine
+FROM python:3.6-alpine
 
 ENV BOT_ROOT=/opt/errbot
 
@@ -9,6 +9,9 @@ RUN apk add --no-cache libffi openssl git \
            gcc \
            libc-dev \
            libffi-dev \
+           libstdc++ \
+           libxml2-dev \
+           libxslt-dev \
            openssl-dev \
     && pip install -r $BOT_ROOT/requirements.txt \
     && pip install slackclient python-telegram-bot \