[CentOS] Installing GCC 5.3.0 in CentOS 6 in Docker.

I need to compile some C++ 11 with GCC 5.3.0 on CentOS 6.  All of this
is happening inside docker in CI using the centos:6 base image.  (NB.
It can't be GCC 6 or 7, because I have to send binaries to clients who
won't install newer vs of libstdc++.so.)

Until recently, I was installing  devtoolset-4-gcc-c++ via yum, and then using

   RUN scl enable devtoolset-4 gcc

to build the code. Unfortunately, devtoolset-4 is no longer supported,
so this has stopped working.

I've been trying to compile GCC from scratch, but it's been running
for hours and hours – even if I get it working, I'll have to replicate
the whole process inside Docker and I suspect it will be very brittle.

Is there any easy way of downloading a precompiled version?  E.g.
-- Is there something that I can do to go back to the old v. of
Software Collections? Or
-- Is there somewhere I might find a precompiled gcc 5 package?

Thanks very much,
Mohan