Add Docker container ID to system metadata

[axw]

Update the "system" metadata with the container ID, if we detect one. Currently we only support doing this from within Docker containers.

Closes #328

[codecov-io]

Codecov Report

Merging #330 into master will decrease coverage by 3.47%.
The diff coverage is 67.85%.

@@            Coverage Diff             @@
##           master     #330      +/-   ##
==========================================
- Coverage    84.5%   81.02%   -3.48%     
==========================================
  Files          96       98       +2     
  Lines        5802     5856      +54     
==========================================
- Hits         4903     4745     -158     
- Misses        830      840      +10     
- Partials       69      271     +202

Impacted Files	Coverage Δ
internal/apmhostutil/container.go	100% <100%> (ø)
utils.go	96.77% <100%> (-1.54%)	⬇️
internal/apmhostutil/docker_linux.go	62.5% <62.5%> (ø)
model/marshal_fastjson.go	72.74% <64.7%> (-9.64%)	⬇️
internal/sqlscanner/token.go	60% <0%> (-20%)	⬇️
utils_linux.go	77.77% <0%> (-11.12%)	⬇️
sampler.go	81.81% <0%> (-9.1%)	⬇️
module/apmsql/utils.go	18.18% <0%> (-9.1%)	⬇️
internal/ringbuffer/buffer.go	81.03% <0%> (-8.63%)	⬇️
... and 33 more

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