Note about docker requirement for tests

[jkwatson]

updates #1067

[codecov-io]

Codecov Report

Merging #1073 into master will increase coverage by 0.05%.
The diff coverage is n/a.

@@             Coverage Diff              @@
##             master    #1073      +/-   ##
============================================
+ Coverage     85.54%   85.59%   +0.05%     
  Complexity     1085     1085              
============================================
  Files           138      138              
  Lines          3993     3993              
  Branches        356      356              
============================================
+ Hits           3416     3418       +2     
+ Misses          435      434       -1     
+ Partials        142      141       -1     

Impacted Files	Coverage Δ	Complexity Δ
...elemetry/sdk/trace/export/BatchSpansProcessor.java	93.23% <0.00%> (+1.50%)	9.00% <0.00%> (ø%)

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