docker push to dockerhub private repo有时会再次推送现有层（ubuntu基础层）

Question

我正推向dockerhub。我以为每次推动它都应该是增量的。但有时它会再次推动现有层。

这里按时间顺序推送很少

// n-th push
1515e3a2aade: Layer already exists 
fbe2db54eaea: Layer already exists 
b3194de9c939: Layer already exists 
abf060fbeb02: Layer already exists 
bc61dcaa1fb9: Layer already exists 
629d3660cc59: Pushed 
5f70bf18a086: Pushed 
0d81735d8272: Pushed 
982549bd6b32: Pushed 
8698b31c92d5: Pushed

// n+1-th push
5a2c20b22bf1: Pushed 
43ba1325889f: Pushed 
c23b65a5145c: Pushed 
abf060fbeb02: Layer already exists 
bc61dcaa1fb9: Layer already exists 
629d3660cc59: Layer already exists 
5f70bf18a086: Layer already exists 
0d81735d8272: Layer already exists 
982549bd6b32: Layer already exists 
8698b31c92d5: Layer already exists

// n+2-th push, notice how layer 8698b31c92d5 and 3 more layers got pushed again
06db42d823c5: Pushed 
a3575dd784b5: Pushed 
e04cb41bff92: Pushed 
abf060fbeb02: Layer already exists 
bc61dcaa1fb9: Layer already exists 
629d3660cc59: Pushed 
5f70bf18a086: Layer already exists 
0d81735d8272: Pushed 
982549bd6b32: Pushed 
8698b31c92d5: Pushed 

这里是n + 2次推送的泊坞历史

54ee4d70917c        10 minutes ago      /bin/sh -c #(nop) EXPOSE 12000/tcp 12001/tcp    0 B                 
8ae93e68a93a        10 minutes ago      /bin/sh -c #(nop) ENTRYPOINT &{["./nexus"]}     0 B                 
7644eba33444        10 minutes ago      /bin/sh -c #(nop) COPY multi:609730bd4dcd2093   571 kB              
ac87f067a414        10 minutes ago      /bin/sh -c #(nop) COPY multi:7b816c50da4eff99   10.84 MB            
52ad31e3d4fb        10 minutes ago      /bin/sh -c #(nop) COPY file:dc5824a16ae1ce7e8   14.9 MB             
841eb28ef938        44 hours ago        /bin/sh -c #(nop) WORKDIR /prod/bin             0 B                 
b167f0cca892        44 hours ago        /bin/sh -c mkdir -p /prod/socketio/asset/       0 B                 
c9c427a1999d        44 hours ago        /bin/sh -c mkdir -p /prod/eval/bin              0 B                 
9bad1ce6d92c        44 hours ago        /bin/sh -c mkdir -p /prod/bin                   0 B                 
3876b81b5a81        3 weeks ago         /bin/sh -c #(nop) CMD ["/bin/bash"]             0 B                 
<missing>           3 weeks ago         /bin/sh -c sed -i 's/^#\s*\(deb.*universe\)$/   1.895 kB            
<missing>           3 weeks ago         /bin/sh -c echo '#!/bin/sh' > /usr/sbin/polic   194.5 kB            
<missing>           3 weeks ago         /bin/sh -c #(nop) ADD file:7ce20ce3daa6af21db   187.7 MB 

知道为什么会再次推送图层<missing> 3 weeks ago /bin/sh -c #(nop) ADD file:7ce20ce3daa6af21db 187.7 MB（8698b31c92d5）吗？

编辑：我重新启动了docker host（docker-machine restart default），这有关系吗？在推送之前，docker push不应该从服务器获取现有层哈希的列表吗？