moby: Permission denied for directories created automatically by Dockerfile ADD command

As part of a Dockerfile I’m running:

ADD id_rsa.pub /home/ubuntu/.ssh/authorized_keys
RUN chown ubuntu:ubuntu -R /home/ubuntu/.ssh

The user account and home directory already exist, so the Docker build process is only creating the .ssh directory. When I log in as ubuntu to the container, no matter what I do I cannot cd to this directory. The permissions and ownership are correct:

$ ls -ld .ssh
drwx------ 2 ubuntu ubuntu 4096 Jul 25 19:26 .ssh

$ whoami
ubuntu

$ id
uid=1000(ubuntu) gid=1000(ubuntu) groups=1000(ubuntu)

$ ls -nd .ssh
drwx------ 2 1000 1000 4096 Jul 25 19:26 .ssh

$ cd .ssh    
-su: cd: .ssh: Permission denied

When I create the directory explicitly before hand then it works as expected:

RUN mkdir /home/ubuntu/.ssh/
RUN chown ubuntu:ubuntu -R /home/ubuntu/.ssh
ADD id_rsa.pub /home/ubuntu/.ssh/authorized_keys

For the life of me I can’t figure out what the difference is. I checked the SELinux context (none) and the Posix ACL (no extended ACLs set).