Include user sshd config drop-ins in the Docker image

What does this MR do?

Adds Include /etc/gitlab/sshd_config.d/*.conf to docker/assets/sshd_config, so users of the Docker image can customize sshd (ciphers, MACs, key exchange algorithms, host certificates) from the persisted /etc/gitlab volume instead of patching /assets/sshd_config.

• If sshd_config.d is missing or empty, nothing changes.
• Default algorithms are unchanged. !8937 (merged) reverted a change to the defaults because it broke existing installations, so this MR gives users a way to set them instead.
• docker/assets/init-container prints a warning if the resulting config fails sshd -t. The existing workaround, appending the same Include from a GITLAB_POST_RECONFIGURE_SCRIPT hook, is described in #8124 (comment 2230870069).

Changelog: added

Related issues

Related to: #8124 (closed)

Test Cases

Using gitlab/gitlab-ce:latest docker image:

Before

Command:

docker run --rm --entrypoint bash gitlab/gitlab-ce:latest -c '
mkdir -p /etc/gitlab/sshd_config.d /run/sshd
for t in rsa ecdsa ed25519; do ssh-keygen -q -f /etc/gitlab/ssh_host_${t}_key -N "" -t $t; done

printf "MACs hmac-sha2-512-etm@openssh.com\nMaxStartups 10:30:100\n" > /etc/gitlab/sshd_config.d/10-test.conf
echo "--- effective values ---"
/usr/sbin/sshd -T -f /assets/sshd_config | grep -iE "^(macs|maxstartups)"

echo "BogusKeyword yes" > /etc/gitlab/sshd_config.d/20-bad.conf
echo "--- bad keyword in drop-in ---"
/usr/sbin/sshd -t -f /assets/sshd_config && echo "sshd -t OK (drop-ins are ignored)"
'

Output:

--- effective values ---
macs umac-64-etm@openssh.com,umac-128-etm@openssh.com,hmac-sha2-256-etm@openssh.com,hmac-sha2-512-etm@openssh.com,hmac-sha1-etm@openssh.com,umac-64@openssh.com,umac-128@openssh.com,hmac-sha2-256,hmac-sha2-512,hmac-sha1
maxstartups 100:30:200
--- bad keyword in drop-in ---
sshd -t OK (drop-ins are ignored)

After

Command:

mkdir -p /tmp/gl-etc/sshd_config.d
printf "MACs hmac-sha2-512-etm@openssh.com\n" > /tmp/gl-etc/sshd_config.d/10-test.conf

docker run -d --name gl-ssh-test -p 2222:22 \
  -v "$PWD/docker/assets/sshd_config:/assets/sshd_config:ro" \
  -v /tmp/gl-etc:/etc/gitlab \
  gitlab/gitlab-ce:latest

docker exec gl-ssh-test /usr/sbin/sshd -T -f /assets/sshd_config | grep -i '^macs'

Output:

macs hmac-sha2-512-etm@openssh.com

Checklist

See Definition of done.

For anything in this list which will not be completed, please provide a reason in the MR discussion.

Required

• MR title and description are up to date, accurate, and descriptive.
• MR targeting the appropriate branch.
• Latest Merge Result pipeline is green.
• When ready for review, MR is labeled workflowready for review per the Distribution MR workflow.

For GitLab team members

If you don't have access to this, the reviewer should trigger these jobs for you during the review process.

• The manual Trigger:ee-package jobs have a green pipeline running against latest commit.
• If config/software or config/patches directories are changed, make sure the build-package-on-all-os job within the Trigger:ee-package downstream pipeline succeeded.
• If you are changing anything SSL related, then the Trigger:package:fips manual job within the Trigger:ee-package downstream pipeline must succeed.
• If CI configuration is changed, the branch must be pushed to dev.gitlab.org to confirm regular branch builds aren't broken.

Expected (please provide an explanation if not completing)

• Test plan indicating conditions for success has been posted and passes.
• Documentation created/updated.
• Tests added.
• Integration tests added to GitLab QA.
• Equivalent MR/issue for the GitLab Chart opened.
• Validate potential values for new configuration settings. Formats such as integer 10, duration 10s, URI scheme://user:passwd@host:port may require quotation or other special handling when rendered in a template and written to a configuration file.