fix(docker): Avoid pip 19.0.1 bug

Why:

 * build was failing as described in pypa/pip#6197

This change addreses the need by:

 * force upgrade of pip as that makes it at least pass on my machine docker
   builds.

Dockerfile

@@ -1,4 +1,4 @@
-FROM python:3.7-alpine
+FROM python:3.6-alpine
 LABEL maintainer="Max Rydahl Andersen <[email protected]>"
 
 WORKDIR /usr/src/app
@@ -8,6 +8,6 @@ COPY . .
 RUN apk add --no-cache --virtual build-dependencies gcc musl-dev\
     &&  rm -rf /var/cache/apk/*
 
-RUN pip3 install --no-cache-dir -e .
+RUN pip3 install --upgrade pip; pip3 install --no-cache-dir -e .
 
 ENTRYPOINT ["hass-cli"]