[centos] use gcc 8 for opencv,mmxnet, and ffmpeg

centos/Dockerfile

@@ -13,6 +13,7 @@ RUN yum install -y \
     yum install -y \
         kernel && \
     yum install -y \
+        devtoolset-8 \
         devtoolset-9 && \
     yum install -y \
         rh-git218-git \
@@ -76,6 +77,7 @@ RUN version=4.2.1 && \
         && \
     yum clean all && \
     source /etc/profile && \
+    source scl_source enable devtoolset-8 && \
     cd /tmp && \
     git clone https://git.videolan.org/git/ffmpeg/nv-codec-headers.git && \
     cd nv-codec-headers && \
@@ -165,6 +167,7 @@ RUN version=4.2.0 && \
         && \
     yum clean all && \
     source /etc/profile && \
+    source scl_source enable devtoolset-8 && \
     cd /tmp && \
     curl -sSL -o opencv-${version}.zip https://github.com/opencv/opencv/archive/${version}.zip && \
     unzip opencv-${version}.zip && \
@@ -223,6 +226,7 @@ ENV MXNet_DIR /opt/mxnet-1.6.0
 ADD patches/mxnet-1.6.0-manual-cuda-arch-flags.patch /opt/mxnet-1.6.0-manual-cuda-arch-flags.patch
 RUN version=1.6.0 && \
     source /etc/profile && \
+    source scl_source enable devtoolset-8 && \
     cd /opt && \
     curl -sSLO https://github.com/apache/incubator-mxnet/releases/download/${version}/apache-mxnet-src-${version}-incubating.tar.gz && \
     tar -xf apache-mxnet-src-${version}-incubating.tar.gz && \