Add tmux to dockerfile

Dockerfile

@@ -13,7 +13,7 @@ RUN apt-get update && apt-get upgrade -y && apt-get install -y \
     make build-essential libssl-dev zlib1g-dev \
     libbz2-dev libreadline-dev libsqlite3-dev wget curl llvm \
     libncursesw5-dev xz-utils tk-dev libxml2-dev libxmlsec1-dev libffi-dev liblzma-dev \
-    vim git \
+    vim git tmux \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*