docker: fix invalid instruction in Dockerfile not using trailing slash for files destination directory

> If the source is a file, and the destination doesn't end with a trailing slash, the source file will be written to the destination path as a file.
~ https://docs.docker.com/reference/dockerfile/#destination

Signed-off-by: Jakub Meysner <git@jakubmeysner.com>

docker/DockerFile.amd64-build

@@ -36,7 +36,7 @@ COPY Makefile Makefile
 RUN make androidsdk
 
 # Preload Gradle
-COPY android/gradlew android/build.gradle android
+COPY android/gradlew android/build.gradle android/
 COPY android/gradle android/gradle
 
 RUN chmod 755 android/gradlew && \