add checks for whether nvidia-docker2 is installed

[tfoote]

Fixes #88

[codecov]

Codecov Report

Merging #103 (8e10651) into master (228125c) will decrease coverage by 2.41%.
The diff coverage is 34.37%.

@@            Coverage Diff             @@
##           master     #103      +/-   ##
==========================================
- Coverage   86.89%   84.48%   -2.42%     
==========================================
  Files           7        7              
  Lines         595      625      +30     
==========================================
+ Hits          517      528      +11     
- Misses         78       97      +19     

Impacted Files	Coverage Δ
src/rocker/nvidia_extension.py	82.10% <13.33%> (-12.90%)	⬇️
src/rocker/core.py	88.88% <52.94%> (-1.98%)	⬇️

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