Retry docker-pulls

[pbrisbin]

Exceptions occur here semi-frequently, for rate limits or transient
network issues it seems. This adds exponential backoff starting with a
base of 1s and retrying up to 5 times.

Example:

Run Seconds Timestamp
0   0.000   00:00:02
1   1.000   00:00:03
2   3.000   00:00:05
3   7.000   00:00:09
4   15.000  00:00:17