Add service image build example

[chris-crone]

- What I did Added an example of a Docker App that has service images that can be built.

- Description for the changelog N/A

[codecov[bot]]

Codecov Report

Merging #723 into master will decrease coverage by 1.45%. The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #723      +/-   ##
==========================================
- Coverage   72.45%   70.99%   -1.46%     
==========================================
  Files          59       59              
  Lines        3438     3031     -407     
==========================================
- Hits         2491     2152     -339     
+ Misses        612      588      -24     
+ Partials      335      291      -44

Impacted Files	Coverage Δ
internal/commands/run.go	60.22% <0%> (-9.86%)	:arrow_down:
internal/commands/remove.go	52% <0%> (-8.57%)	:arrow_down:
internal/store/bundle.go	68.93% <0%> (-7.75%)	:arrow_down:
internal/commands/update.go	58.18% <0%> (-7.21%)	:arrow_down:
internal/cnab/cnab.go	63.33% <0%> (-6.24%)	:arrow_down:
internal/commands/image/list.go	83.33% <0%> (-5.82%)	:arrow_down:
types/parameters/parameters.go	93.65% <0%> (-3.18%)	:arrow_down:
internal/commands/push.go	45.22% <0%> (-2.57%)	:arrow_down:
internal/commands/image/tag.go	88.88% <0%> (-1.31%)	:arrow_down:
internal/store/digest.go	86.36% <0%> (-0.82%)	:arrow_down:
... and 1 more

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[chris-crone]

I still need to add a readme for this so will leave it in draft until that's done.

[ndeloof]

https://github.com/docker/app/tree/master/examples/voting-app is already such a docker app build sample, maybe it should have better documentation to make it visible ?