Docker: Fix sharp not installing due to npm cache permissions

Moving the npm cache folder into the /app folder solved the permission issues
Fixes #8

.gitignore

@@ -19,6 +19,7 @@ yarn-debug.log*
 yarn-error.log*
 lerna-debug.log*
 .pnpm-debug.log*
+.npm_cache
 
 # Diagnostic reports (https://nodejs.org/api/report.html)
 report.[0-9]*.[0-9]*.[0-9]*.[0-9]*.json