Simplify dockerfile

[ocefpaf]

@benjwadams after our debug session yesterday I decided to take a look into the Dockerfile and made a few modifications. In this PR:

• we rely more heavily on Python wheels
• update the centos image so we can use at least python 3.8
• simplify and update the requirements.txt
• fix the deprecated MAINTAINER syntax in Dockerfiles

We can probably simplify this further by reducing the layers and removing the yum installation of epel-release and redis. The former I'm not sure we need and the latter is duplicated from the docker composition later.

Bare in mind that I did not test the actual deployment, just the build!

[benjwadams]

@ocefpaf nice work. I made a PR against your branch so that the Dockerfile runs properly under Python 3.

I'm also having a few issues with importing the RQ library, which although is specified in the requirements file, is not importing properly, possibly due to path issues. I'll see if I can resolve those prior to merging.

EDIT: Above is fixed in PR against your branch.

[ocefpaf]

Thanks @benjwadams! Let me know if this is good to go.

I'll play with moving the setup to conda in the future to see if we can simplify it a bit further and reduce the size of the image.