Capitalise help outputs

[chris-crone]

- What I did
Fix #194.

- How I did it
Manually run through commands and verify help outputs.

- How to verify it
Manually run through commands and verify help outputs.

- Description for the changelog
N/A

[vdemeester]

LGTM 🐯

[codecov]

Codecov Report

Merging #349 into master will increase coverage by 0.52%.
The diff coverage is 100%.

@@            Coverage Diff            @@
##           master    #349      +/-   ##
=========================================
+ Coverage   58.18%   58.7%   +0.52%     
=========================================
  Files          58      56       -2     
  Lines        3178    2802     -376     
=========================================
- Hits         1849    1645     -204     
+ Misses       1071     937     -134     
+ Partials      258     220      -38

Impacted Files	Coverage Δ
cmd/docker-app/push.go	86.66% <100%> (-8.79%)	⬇️
cmd/docker-app/save.go	88.88% <100%> (ø)	⬆️
cmd/docker-app/root.go	76.78% <100%> (+0.11%)	⬆️
cmd/docker-app/deploy.go	34.88% <100%> (ø)	⬆️
internal/packager/fork.go	70.14% <0%> (-10.58%)	⬇️
internal/packager/registry.go	72.82% <0%> (-9.53%)	⬇️
internal/packager/extract.go	65.33% <0%> (-5.95%)	⬇️
cmd/docker-app/pull.go	0% <0%> (ø)	⬆️
... and 4 more

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