How do I declare multiple maintainers in my Dockerfile?

You can only specify one MAINTAINER instruction in a Dockerfile.

Furthermore, MAINTAINER will be deprecated in the upcoming 1.13.0 release, see deprecations and this pull request.

The recommended solution is to use LABEL instead, e.g.

LABEL authors="first author,second author"

Labels have a key=value syntax. This means you cannot assign the same label more than once and you cannot assign multiple values to a given label. But you can combine multiple values into one with a syntax of your choice as illustrated in the example..