Minify Docker images

As in #67 (closed), now for the following new images:

• ALFA v1.1.1: image (167.18 MB) / Dockerfile
• bedGraphToBigWig v4: image (30.29 MB) / Dockerfile
• ImageMagick v6.9.10: image (66.54 MB) / Dockerfile
• MultiQC v1.7: keep on using maintainer's image (56.39 MB) / unversioned Dockerfile
• gffread v0.11.7: added -slim tag to image (26.1 MB) for consistency / Dockerfile