added pip to Dockerfile due to install of mkdocs
added pip to Dockerfile due to install of mkdocs

nimishzynga/cbgt,steveyen/cbft,couchbaselabs/cbft,steveyen/cbft,nimishzynga/cbgt,steveyen/cbft,nimishzynga/cbgt,couchbaselabs/cbft,couchbaselabs/cbgt,nimishzynga/cbgt,couchbaselabs/cbgt,steveyen/cbft,couchbaselabs/cbgt,couchbaselabs/cbgt,couchbaselabs/cbft,couchbaselabs/cbft,steveyen/cbft