MySQL 5.5 was only supported on Debian 6, and since that's now eol we can't expect any new 5.5 packages to be released for any Debian/Ubuntu platforms.

A few options:
Keep it at the current version:
Not a good idea, since no more updates will be released, security or otherwise

Change to a different base image:
Oracle's own Docker images use OracleLinux as a base, and it will still get packages. The base image is much larger than Debian's, but currently this won't affect the total size much (actually make it smaller) since we use a custom rpm for Docker with various things removed. Main disadvantage is that it would break any images that are based on the image and then do anything deb-specific (e.g. apt). There are also some differences in the default config and file locations (my.cnf is in /etc/ instead of /etc/mysql/) that we'd need to deal with.

Drop it:
Do we know how much use there is of the 5.5 image? Is there any way to see download figures for a single tag?