Check changelog generator docker image reference

[philwebb]

CI for the RC1 promote gives:

fetching harbor-mirror.spring.vmware.com/springio/github-changelog-generator@sha256:c6eec12b5ecb6efa32b07d4e7ea433a682f316b46675d9d920c7bda66bece7f1

WARN[0000] download from mirror harbor-mirror.spring.vmware.com/springio/github-changelog-generator failed: get image: unrecognized challenge:  

fetching springio/github-changelog-generator@sha256:c6eec12b5ecb6efa32b07d4e7ea433a682f316b46675d9d920c7bda66bece7f1

WARN[0001] too many requests; retrying in 5.52330144s   
WARN[0008] too many requests; retrying in 10.803818161s 
WARN[0020] too many requests; retrying in 13.101300599s 
WARN[0034] too many requests; retrying in 15.823926909s 
WARN[0051] too many requests; retrying in 23.404886645s 
WARN[0075] too many requests; retrying in 45.062188548s 
WARN[0121] too many requests; retrying in 32.21479586s  
WARN[0154] too many requests; retrying in 56.086234769s 
WARN[0211] too many requests; retrying in 1m16.498379171s 
WARN[0289] too many requests; retrying in 2m33.948712421s