Dockerfiles can be affected by bad design choices, known as Dockerfile smells. Hadolint is currently the reference tool able to detect them, and it is widely used both by researchers and practitioners. The literature shows that these smells are commonly diffused in Dockerfiles, but it is still not clear how developers perceive them as bad practices to avoid. This paper aims to investigate the relevance of Dockerfile smells from the perspective of expert Dockerfile developers. We first perform a mining-based study in which we mine the change history of Dockerfiles maintained by experts to understand what smells have been more frequently introduced in their history. Next, we ran a survey where we asked expert Dockerfile developers to evaluate three different Dockerfiles affected each one by a different smell. We obtained a total of 94 responses for 17 smells, representative of 24 Dockerfile smells. Expert Dockerfile developers prioritize a small part of the evaluated smells over others. Besides, they report several other bad practices not captured by the existing smells. This allowed us to propose a ranked catalog containing 26 additional Dockerfile smells, which can be used as a guide for novices to understand which aspects to focus on to write good-quality Dockerfiles.