Add curl to the docker image

[MattDietz]

The etcd-start script will fetch etcd if no archive is present, but curl
has to be installed into the container for that to actually work.

[coveralls]

Pull Request Test Coverage Report for Build 140

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 39 unchanged lines in 3 files lost coverage.
• Overall coverage decreased (-2.8%) to 76.587%

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Files with Coverage Reduction	New Missed Lines	%
etcd3/_utils.py	1	90.91%
etcd3/_grpc_stubs/rpc_pb2.py	16	71.77%
etcd3/_client.py	22	97.3%

Totals
Change from base Build 137:	-2.8%
Covered Lines:	1086
Relevant Lines:	1418

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💛 - Coveralls