[docs][docker] KUDU-3159 Fix missing devtoolset3 package on el6

devtoolset3 package is no longer available on software collections causing
build failures when building kudu from source on RHEL/CentOS 6.

Using one of the only available unofficial copr repositories to install
devtoolset3 since devtoolset3 is EOL already and we plan to migrate to a newer
compiler version and hence a newer devtoolset for RHEL/CentOS 6 soon.

Separate JIRA KUDU-3160 tracks upgrading devtoolset version for el6.

Tests:
- Successfully ran kudu-unit tests using devtoolset3 downloaded from the repo
  on an internal cloudera test job.
- Verified doc instructions using ascii doc plugin in CLion IDE
- Partially verified build with docker BASES=centos:6 docker/docker-build.sh
on Mac OS 10.15.4
Verified from console that devtoolset3 repo was installed.
However ran into issue while building kudu-python from what looks
like an unrelated issue due to failure to find kudu_client library.

Change-Id: I9c4b9546c835df26f993ed6164e92f2d4c55c3fe
Reviewed-on: http://gerrit.cloudera.org:8080/16114
Tested-by: Kudu Jenkins
Reviewed-by: Grant Henke <[email protected]>

docker/bootstrap-dev-env.sh

@@ -100,12 +100,10 @@ if [[ -f "/usr/bin/yum" ]]; then
   # To build on a version older than 7.0, the Red Hat Developer Toolset
   # must be installed (in order to have access to a C++11 capable compiler).
   if [[ "$OS_MAJOR_VERSION" -lt "7" ]]; then
-    DTLS_RPM=rhscl-devtoolset-3-epel-6-x86_64-1-2.noarch.rpm
-    DTLS_RPM_URL=https://www.softwarecollections.org/repos/rhscl/devtoolset-3/epel-6-x86_64/noarch/${DTLS_RPM}
-    wget ${DTLS_RPM_URL} -O ${DTLS_RPM}
-    yum install -y scl-utils ${DTLS_RPM}
+    DTLS_REPO_URL=https://copr.fedorainfracloud.org/coprs/rhscl/devtoolset-3/repo/epel-6/rhscl-devtoolset-3-epel-6.repo
+    yum install -y scl-utils yum-utils
+    yum-config-manager --add-repo=${DTLS_REPO_URL}
     yum install -y devtoolset-3-toolchain
-    rm -f $DTLS_RPM
   fi
 
   # Reduce the image size by cleaning up after the install.

docs/installation.adoc

@@ -103,10 +103,9 @@ $ sudo yum install autoconf automake cyrus-sasl-devel cyrus-sasl-gssapi \
 Toolset.
 +
 ----
-$ DTLS_RPM=rhscl-devtoolset-3-epel-6-x86_64-1-2.noarch.rpm
-$ DTLS_RPM_URL=https://www.softwarecollections.org/repos/rhscl/devtoolset-3/epel-6-x86_64/noarch/${DTLS_RPM}
-$ wget ${DTLS_RPM_URL} -O ${DTLS_RPM}
-$ sudo yum install -y scl-utils ${DTLS_RPM}
+$ DTLS_REPO_URL=https://copr.fedorainfracloud.org/coprs/rhscl/devtoolset-3/repo/epel-6/rhscl-devtoolset-3-epel-6.repo
+$ sudo yum install -y scl-utils yum-utils
+$ sudo yum-config-manager --add-repo=${DTLS_REPO_URL}
 $ sudo yum install -y devtoolset-3-toolchain
 ----
 
@@ -230,10 +229,9 @@ sudo yum -y install autoconf automake curl cyrus-sasl-devel cyrus-sasl-gssapi \
   cyrus-sasl-plain flex gcc gcc-c++ gdb git java-1.8.0-openjdk-devel \
   krb5-server krb5-workstation libtool make openssl-devel patch pkgconfig \
   redhat-lsb-core rsync unzip vim-common which
-DTLS_RPM=rhscl-devtoolset-3-epel-6-x86_64-1-2.noarch.rpm
-DTLS_RPM_URL=https://www.softwarecollections.org/repos/rhscl/devtoolset-3/epel-6-x86_64/noarch/${DTLS_RPM}
-wget ${DTLS_RPM_URL} -O ${DTLS_RPM}
-sudo yum install -y scl-utils ${DTLS_RPM}
+DTLS_REPO_URL=https://copr.fedorainfracloud.org/coprs/rhscl/devtoolset-3/repo/epel-6/rhscl-devtoolset-3-epel-6.repo
+sudo yum install -y scl-utils yum-utils
+sudo yum-config-manager --add-repo=${DTLS_REPO_URL}
 sudo yum install -y devtoolset-3-toolchain
 git clone https://github.com/apache/kudu
 cd kudu