Outdated docs for Centos installation, possibly not working for Centos 8

[vlizanae]

Relevant system information:

• OS: Building Centos 8.1.1911 based Docker image.
• PostgreSQL version (output of postgres --version): 11.*
• TimescaleDB version (output of \dx in psql): latest possible
• Installation method: rpm

Describe the bug I'm building an image based on this documentation.

• First issue was that timescale_timescaledb.repo file shown was outdated, had to go into the repo to find out there is a script for rpm repo installation that is not mentioned anywhere on the docs.
• Installed correct pgdg for Centos 8 as well as timescale repo via script, then yum install timescaledb-postgresql-11 fails with the following error:

  Error: 
  Problem: package timescaledb-postgresql-11-1.4.2-0.el8.x86_64 requires postgresql11-server >= 11.4, but none of the providers can be installed
  - conflicting requests
  - package postgresql11-server-11.4-1PGDG.rhel8.x86_64 is excluded
  - package postgresql11-server-11.5-1PGDG.rhel8.x86_64 is excluded
  - package postgresql11-server-11.6-1PGDG.rhel8.x86_64 is excluded
  - package postgresql11-server-11.6-2PGDG.rhel8.x86_64 is excluded
  - package postgresql11-server-11.7-1PGDG.rhel8.x86_64 is excluded
  - package postgresql11-server-11.8-1PGDG.rhel8.x86_64 is excluded

To Reproduce Steps to reproduce the behavior:

1. Create a Dockerfile:

   
   FROM centos:8

RUN yum install -y https://download.postgresql.org/pub/repos/yum/reporpms/EL-8-x86_64/pgdg-redhat-repo-latest.noarch.rpm RUN curl -s https://packagecloud.io/install/repositories/timescale/timescaledb/script.rpm.sh | bash RUN yum install -y timescaledb-postgresql-11

2. `docker build .`

**Expected behavior**
Image should complete building process.

**Actual behavior**
Building process fails at `yum install -y timescaledb-postgresql-11`.

[svenklemm]

you might have to disable the system postgres dnf -qy module disable postgresql

[svenklemm]

See https://github.com/timescale/timescaledb/pull/2050/files for installation of packages on centos 8