Struggling to release:perform with git and deployer key

Hi all -

I'm really struggling with this.

My git push portion of the maven release process just is quitting with this:

00:01:29.803 Provider message:
00:01:29.803 The git-push command failed.
00:01:29.803 Command output:
00:01:29.803 Permission denied (publickey).
00:01:29.803 fatal: The remote end hung up unexpectedly
00:01:29.803
00:01:29.803 cause : Unable to commit files
00:01:29.803 Provider message:
00:01:29.803 The git-push command failed.
00:01:29.803 Command output:
00:01:29.803 Permission denied (publickey).
00:01:29.803 fatal: The remote end hung up unexpectedly

I have the key stored in Jenkins as a "managed file" (as part of that
plugin). So the build has an early step where that key is written to
disk.

Then I've used the settings.xml to specify which key to use (pointing
at that location just above).

<server>
<id>github.com</id>
<username>someusername</username>
<privateKey>/srv/jenkins/.ssh/somekeyname</privateKey>
<filePermissions>664</filePermissions>
<directoryPermissions>775</directoryPermissions>
<configuration></configuration>
</server>

<scm>
<connection>scm:git:git [ at ] github.com:${companyid}/${project.artifactId}.git</connection>
<url>scm:git:git [ at ] github.com:${companyid}/${project.artifactId}.git</url>
<developerConnection>scm:git:git [ at ] github.com:${companyid}/${project.artifactId}.git</developerConnection>
<tag>HEAD</tag>
</scm>

What am I doing wrong?