Update docker-ce.repo to not hardcode $releasever=7

This keeps it in line with https://download.docker.com/linux/centos/docker-ce.repo

Whether or not Docker works well on CentOS 8 for our purposes
hasn't been verified yet.

Related to #300

roles/matrix-base/files/yum.repos.d/docker-ce.repo

@@ -1,62 +1,62 @@
 [docker-ce-stable]
 name=Docker CE Stable - $basearch
-baseurl=https://download.docker.com/linux/centos/7/$basearch/stable
+baseurl=https://download.docker.com/linux/centos/$releasever/$basearch/stable
 enabled=1
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
 [docker-ce-stable-debuginfo]
 name=Docker CE Stable - Debuginfo $basearch
-baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/stable
+baseurl=https://download.docker.com/linux/centos/$releasever/debug-$basearch/stable
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
 [docker-ce-stable-source]
 name=Docker CE Stable - Sources
-baseurl=https://download.docker.com/linux/centos/7/source/stable
+baseurl=https://download.docker.com/linux/centos/$releasever/source/stable
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
-[docker-ce-edge]
-name=Docker CE Edge - $basearch
-baseurl=https://download.docker.com/linux/centos/7/$basearch/edge
+[docker-ce-test]
+name=Docker CE Test - $basearch
+baseurl=https://download.docker.com/linux/centos/$releasever/$basearch/test
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
-[docker-ce-edge-debuginfo]
-name=Docker CE Edge - Debuginfo $basearch
-baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/edge
+[docker-ce-test-debuginfo]
+name=Docker CE Test - Debuginfo $basearch
+baseurl=https://download.docker.com/linux/centos/$releasever/debug-$basearch/test
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
-[docker-ce-edge-source]
-name=Docker CE Edge - Sources
-baseurl=https://download.docker.com/linux/centos/7/source/edge
+[docker-ce-test-source]
+name=Docker CE Test - Sources
+baseurl=https://download.docker.com/linux/centos/$releasever/source/test
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
-[docker-ce-test]
-name=Docker CE Test - $basearch
-baseurl=https://download.docker.com/linux/centos/7/$basearch/test
+[docker-ce-nightly]
+name=Docker CE Nightly - $basearch
+baseurl=https://download.docker.com/linux/centos/$releasever/$basearch/nightly
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
-[docker-ce-test-debuginfo]
-name=Docker CE Test - Debuginfo $basearch
-baseurl=https://download.docker.com/linux/centos/7/debug-$basearch/test
+[docker-ce-nightly-debuginfo]
+name=Docker CE Nightly - Debuginfo $basearch
+baseurl=https://download.docker.com/linux/centos/$releasever/debug-$basearch/nightly
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg
 
-[docker-ce-test-source]
-name=Docker CE Test - Sources
-baseurl=https://download.docker.com/linux/centos/7/source/test
+[docker-ce-nightly-source]
+name=Docker CE Nightly - Sources
+baseurl=https://download.docker.com/linux/centos/$releasever/source/nightly
 enabled=0
 gpgcheck=1
 gpgkey=https://download.docker.com/linux/centos/gpg