Dockerfile: update to latest debian-base

[BenTheElder]

Kubernetes v1.17.0 shipped using debian-base:v2.0.0, by upgrading to match we can de-dupe base image layer requirements.

[BenTheElder]

cc @wenjiaswe xref: #10805

[codecov-io]

Codecov Report

Merging #11471 into master will decrease coverage by 0.23%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master   #11471      +/-   ##
==========================================
- Coverage   64.21%   63.98%   -0.24%     
==========================================
  Files         403      403              
  Lines       38079    38079              
==========================================
- Hits        24453    24364      -89     
- Misses      11993    12060      +67     
- Partials     1633     1655      +22

Impacted Files	Coverage Δ
client/client.go	47.71% <0%> (-36.28%)	⬇️
client/keys.go	56.78% <0%> (-34.68%)	⬇️
etcdserver/api/v3rpc/lease.go	67.04% <0%> (-7.96%)	⬇️
pkg/netutil/netutil.go	63.11% <0%> (-7.38%)	⬇️
clientv3/namespace/watch.go	87.87% <0%> (-6.07%)	⬇️
clientv3/concurrency/mutex.go	62.16% <0%> (-5.41%)	⬇️
clientv3/leasing/util.go	95% <0%> (-3.34%)	⬇️
proxy/grpcproxy/watch.go	89.94% <0%> (-2.96%)	⬇️
etcdserver/api/v3election/election.go	61.11% <0%> (-2.78%)	⬇️
clientv3/balancer/balancer.go	86.36% <0%> (-2.28%)	⬇️
... and 24 more

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