Add warnings for env_file entries not copied

[ulyssessouza]

As 'env_file' entries are not taken in account,
we warn the user to copy that and fix it's
compose file.

- What I did
Add a warning for each env_file entry on each service

- How I did it
By checking the env_file block for each service entry in docker app init

- How to verify it
Create a docker-compose.yml file with:

version: '3.7'
services:
  front:
    image: "nginx"
    env_file:
     - "myenv.env1"
     - "myenv.env2"

Run:

docker app init --compose-file docker-compose.yml myapp

- Description for the changelog
Add warnings for env_file entries not copied

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

[codecov]

Codecov Report

Merging #610 into master will decrease coverage by 0.03%.
The diff coverage is 70.9%.

@@            Coverage Diff             @@
##           master     #610      +/-   ##
==========================================
- Coverage   70.57%   70.53%   -0.04%     
==========================================
  Files          62       62              
  Lines        3208     3248      +40     
==========================================
+ Hits         2264     2291      +27     
- Misses        639      647       +8     
- Partials      305      310       +5

Impacted Files	Coverage Δ
internal/commands/init.go	86.66% <100%> (ø)	⬆️
internal/packager/init.go	63.26% <70.37%> (+1.58%)	⬆️

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[GordonTheTurtle]

Please sign your commits following these rules:
https://github.com/moby/moby/blob/master/CONTRIBUTING.md#sign-your-work
The easiest way to do this is to amend the last commit:

$ git clone -b "add-env_file-warning" git@github.com:ulyssessouza/app.git somewhere
$ cd somewhere
$ git rebase -i HEAD~842354452128
editor opens
change each 'pick' to 'edit'
save the file and quit
$ git commit --amend -s --no-edit
$ git rebase --continue # and repeat the amend for each commit
$ git push -f

Amending updates the existing PR. You DO NOT need to open a new one.

[silvin-lubecki]

LGTM with small typo fixed