Adjust the "docker app build" UX

[ndeloof]

- What I did Amended the docker app build user experience: tag is optional (set by --tag option) a context path is required (aligned with docker build)

- How I did it service image get tagged, based on metadata or --tag option

- How to verify it docker app build --t foo:1 . will tag built services images as foo:1-service and invocation image as foo-1-installer

- Description for the changelog Service images get tagged during build to match --tag option or metadata

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

[codecov[bot]]

Codecov Report

Merging #691 into master will decrease coverage by 0.09%. The diff coverage is n/a.

@@            Coverage Diff            @@
##           master     #691     +/-   ##
=========================================
- Coverage   72.51%   72.41%   -0.1%     
=========================================
  Files          56       56             
  Lines        3002     3031     +29     
=========================================
+ Hits         2177     2195     +18     
- Misses        552      559      +7     
- Partials      273      277      +4

Impacted Files	Coverage Δ
internal/commands/build/compose.go	80.43% <0%> (-12.02%)	:arrow_down:
internal/commands/build/build.go	71.35% <0%> (-5.01%)	:arrow_down:
internal/commands/update.go
internal/commands/upgrade.go	58.18% <0%> (ø)
internal/store/digest.go	90.47% <0%> (+7.14%)	:arrow_up:
internal/commands/image/list.go	89% <0%> (+7.18%)	:arrow_up:

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