Update docker images for 1.5.x

[cbhp]

Can you please provide updated docker images for 1.5.x as well?

Last docker image seems to be 1.5.11 whereas 1.5.14 is released in meantime:
https://hub.docker.com/r/roundcube/roundcubemail/tags?name=1.5

[pabzm]

The build process for v1.5 unfortunately is broken since a while (probably due to changes in the upstream image, e.g.), but with v1.7 at the doorstep (also since quite a while) I don't want to invest more time that I did spend on it, since with v1.7 published, 1.5.x will be end of life anyway.

If someone would contribute a fix to the build process, I'd take it though!