Chown files to a custom user after building (fix file ownership) (#250)

* Chown files to a custom user after building

* Better describe `chownFilesTo` input

As suggested by @webbertakken: https://github.com/game-ci/unity-builder/pull/250#discussion_r624575666

Co-authored-by: Webber Takken <webber@takken.io>

* Simplify chown step in `build.sh`

Co-authored-by: Webber Takken <webber@takken.io>

Co-authored-by: Webber Takken <webber@takken.io>

src/model/docker.ts

@@ -36,6 +36,7 @@ class Docker {
       androidKeyaliasName,
       androidKeyaliasPass,
       customParameters,
+      chownFilesTo,
     } = parameters;
 
     const command = `docker run \
@@ -62,6 +63,7 @@ class Docker {
         --env ANDROID_KEYALIAS_NAME="${androidKeyaliasName}" \
         --env ANDROID_KEYALIAS_PASS="${androidKeyaliasPass}" \
         --env CUSTOM_PARAMETERS="${customParameters}" \
+        --env CHOWN_FILES_TO="${chownFilesTo}" \
         --env GITHUB_REF \
         --env GITHUB_SHA \
         --env GITHUB_REPOSITORY \