dockerfile: drop inline 'dotnet nuget add source isn.pschneider.fr'

The project-level NuGet.config (added in 94012c51) lists the isn feed
so 'dotnet restore' picks it up without an inline 'dotnet nuget add
source' step.

The inline add source was duplicating NuGet.config and causing build
failures in GitHub Actions:
  - The --allow-insecure-connections flag did not match the actual
    HTTPS deployment of isn.pschneider.fr (Letsencrypt-issued cert,
    not self-signed), making the step fail with 'exit code 1'.
  - docker build --target build-env (used by
    .github/workflows/docker-publish-android.yml) hit this on every
    run.

Both Dockerfile and Dockerfile.backend had the same redundant step;
both removed. 'dotnet restore' still finds the feed via NuGet.config
at /src/NuGet.config (copied in by 'COPY . .').

Dockerfile

@@ -46,10 +46,6 @@ COPY src/PostIt/PostIt.Desktop/*.csproj ./src/PostIt/PostIt.Desktop/
 # (2) Tout le code source
 COPY . .
 
-# (3) Source NuGet interne (Letsencrypt, certificat auto-signé côté
-# serveur, justifié par build privé).
-RUN dotnet nuget add source https://isn.pschneider.fr/api/v3/index.json --allow-insecure-connections
-
 # (4) Restore
 RUN dotnet restore