Demonstrate docker app build using dockercoins sample app

[ndeloof]

- What I did
Added a docker app example based on dockercoins

- How I did it
Had to tweak the docker-compose file a few

- How to verify it
run docker app build coins from the examples/dockercoins folder

- Description for the changelog
Added example dockerapp demonstrating support for build

- A picture of a cute animal (not mandatory but encouraged)

[codecov]

Codecov Report

Merging #684 into master will decrease coverage by 0.94%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master     #684      +/-   ##
==========================================
- Coverage   72.29%   71.34%   -0.95%     
==========================================
  Files          58       56       -2     
  Lines        3140     2886     -254     
==========================================
- Hits         2270     2059     -211     
+ Misses        581      563      -18     
+ Partials      289      264      -25

Impacted Files	Coverage Δ
internal/commands/build/build.go	58.26% <0%> (-7.79%)	⬇️
internal/packager/bundle.go	61.53% <0%> (-5.71%)	⬇️
types/parameters/parameters.go	92.06% <0%> (-4.77%)	⬇️
internal/commands/cnab.go	71.36% <0%> (-4.51%)	⬇️
internal/commands/install.go	65.71% <0%> (-4.5%)	⬇️
internal/commands/image/rm.go	62.5% <0%> (-1.14%)	⬇️
internal/commands/image/tag.go	89.18% <0%> (-0.56%)	⬇️
internal/store/bundle.go	73.56% <0%> (-0.54%)	⬇️
internal/store/digest.go
internal/commands/run.go

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[silvin-lubecki]

LGTM