render command renders App image only

[jcsirot]

- What I did This PR ensure that the docker app render takes a App image reference argument and fails when a .dockerapp directory is used. If the image is not present locally in the bundle store, the App image is automatically fetched.

- How to verify it Run docker app render myapp.dockerapp and verify it fails with an error indicating that App must be built first. Run docker app render myapp:mytag --set param=some_value and verify the app is correctly rendered.

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

[codecov[bot]]

Codecov Report

Merging #724 into master will increase coverage by 0.32%. The diff coverage is 94.44%.

@@            Coverage Diff             @@
##           master     #724      +/-   ##
==========================================
+ Coverage   71.49%   71.82%   +0.32%     
==========================================
  Files          59       59              
  Lines        3280     3329      +49     
==========================================
+ Hits         2345     2391      +46     
- Misses        605      606       +1     
- Partials      330      332       +2

Impacted Files	Coverage Δ
internal/names.go	100% <ø> (ø)	:arrow_up:
internal/commands/run.go	70.54% <100%> (+0.46%)	:arrow_up:
internal/commands/image/list.go	83.33% <100%> (ø)	:arrow_up:
internal/packager/cnab.go	97.91% <100%> (+0.18%)	:arrow_up:
internal/commands/parameters.go	86.51% <84.61%> (-0.33%)	:arrow_down:
internal/commands/render.go	76.71% <0%> (+5.8%)	:arrow_up:

---

Continue to review full report at Codecov.

Legend - Click here to learn more Δ = absolute <relative> (impact), ø = not affected, ? = missing data Powered by Codecov. Last update cbf819c...c2f76a8. Read the comment docs.