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

Edited Mar 21, 2020 by Alex Kanitz

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