Fix docker build (#4358)

/app/gitea/gitea is the file, and /usr/local/bin is where the symlink will be located

Dockerfile

@@ -58,4 +58,4 @@ CMD ["/bin/s6-svscan", "/etc/s6"]
 
 COPY docker /
 COPY --from=build-env /go/src/code.gitea.io/gitea/gitea /app/gitea/gitea
-RUN ln -s /usr/local/bin/gitea /app/gitea/gitea
+RUN ln -s /app/gitea/gitea /usr/local/bin/gitea