Fix docker build failure (#1179)

* Fix docker build failure

Fix the docker build error from https://github.com/esphome/issues/issues/2041

* Switch from ubuntu to python-slim 

Using python:slim  instead of python reduces the image size from ~1.1 GB to 410MB

Dockerfile

@@ -1,22 +1,18 @@
-FROM ubuntu:xenial
+FROM python:slim
 
 RUN apt-get update && apt-get install -y --no-install-recommends \
         curl \
-        python3 \
-        python3-pip \
-        python3-setuptools \
-        python3-pygments \
         git \
         make \
         doxygen \
         openssh-client \
         software-properties-common \
-    && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/*
+        && apt-get clean && rm -rf /var/lib/apt/lists/* /tmp/*
 
 RUN pip3 install --no-cache-dir --no-binary :all: \
         sphinx
 
 EXPOSE 8000
-WORKDIR /data/esphome-docs
+WORKDIR /data/esphomedocs
 
 CMD ["make", "webserver"]