Update to Dockerfile used for releasing 2.8 and 2.8.1 (#3966)

Dockerfile

@@ -4,13 +4,16 @@ RUN mkdir /kubespray
 WORKDIR /kubespray
 RUN apt update -y && \
     apt install -y \
-    libssl-dev python-dev sshpass apt-transport-https \
+    libssl-dev python-dev sshpass apt-transport-https jq \
     ca-certificates curl gnupg2 software-properties-common python-pip
-RUN curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add - && \
-    add-apt-repository \
-    "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
-    $(lsb_release -cs) \
-    stable" \
-    && apt update -y && apt-get install docker-ce -y
+RUN  curl -fsSL https://download.docker.com/linux/ubuntu/gpg | apt-key add - && \
+     add-apt-repository \
+     "deb [arch=amd64] https://download.docker.com/linux/ubuntu \
+     $(lsb_release -cs) \
+     stable" \
+     && apt update -y && apt-get install docker-ce -y
 COPY . .
 RUN /usr/bin/python -m pip install pip -U && /usr/bin/python -m pip install -r tests/requirements.txt && python -m pip install -r requirements.txt
+RUN curl -LO https://storage.googleapis.com/kubernetes-release/release/v1.11.3/bin/linux/amd64/kubectl \
+    && chmod a+x kubectl && cp kubectl /usr/local/bin/kubectl
+