/etc/passwd is read only?

I’m creating swarms in AWS and trying to modify my /etc/passwd. I don’t get why I can’t though, since I am root and perms are fine:
/home/docker # ls -la /etc/passwd
-rw-r--r-- 1 root root 1389 Jul 22 23:47 /etc/passwd
/home/docker # whoami
root

I must be missing something obvious. I am ssh’ing to my manager node for this.