Pin pip to 18.1 till the bug described there pypa/pip#6158 is not fixed

Dockerfile

@@ -14,7 +14,7 @@ ADD "https://raw.githubusercontent.com/home-assistant/home-assistant/${VERSION}/
 RUN apk add --no-cache git python3 ca-certificates libffi-dev libressl-dev nmap iputils && \
     addgroup -g ${GUID} hass && \
     adduser -h /data -D -G hass -s /bin/sh -u ${UID} hass && \
-    pip3 install --upgrade --no-cache-dir pip && \
+    pip3 install --upgrade --no-cache-dir pip==18.1 && \
     apk add --no-cache --virtual=build-dependencies build-base linux-headers python3-dev tzdata && \
     cp "/usr/share/zoneinfo/${TIMEZONE}" /etc/localtime && echo "${TIMEZONE}" > /etc/timezone && \
     sed '/^$/q' /tmp/requirements_all.txt > /tmp/requirements_core.txt && \