In your console output it says: Maybe run: yum groups mark install (see man yum)—did you do this?

Try running the following commands:

yum groups mark install "Development Tools"
yum groups mark convert "Development Tools"
yum groupinstall "Development Tools"

Reference: RedHat Customer Portal discussion

This issue in this case is RedHat subscription channels. Though the subscription and software are free (provided you have an active subscription already), for some reason you have to make a 'special request' to RedHat as per:

https://access.redhat.com/solutions/472793

You can automate this by visiting:

https://www.redhat.com/wapps/try/RHSCL

(when logged into the support portal). This should automatically be approved after which you can attach a new subscription. Identify the pool id using:

subscription-manager list --available --all

To find the pool id:

 Subscription Name:   Software Collections and Developer Toolset
 Provides:            Red Hat Developer Tools (for RHEL Server)
                      Red Hat Developer Tools Beta (for RHEL Workstation)
                      Red Hat Software Collections Beta (for RHEL Server)
                      Red Hat Software Collections Beta (for RHEL Workstation)
                      Red Hat Software Collections (for RHEL Server)
                      Red Hat Developer Tools Beta (for RHEL Server)
                      Red Hat Developer Toolset (for RHEL Workstation)
                      Red Hat Software Collections Beta (for RHEL Client)
                      Red Hat Software Collections (for RHEL Workstation)
                      Red Hat Developer Toolset (for RHEL Server)
                      Red Hat Developer Tools (for RHEL Workstation)
                      Red Hat Software Collections (for RHEL Client) 
SKU:                 foobar 
Contract:            1234 Pool ID: XXXXXXXXXX

Then attach this and enable the newly available repos:

>subscription-manager attach --pool=XXXXXXXXXXXXXXXXXXXX
Successfully attached a subscription for: Software Collections and Developer Toolset
>subscription-manager repos --enable rhel-workstation-rhscl-7-rpms
Repository 'rhel-workstation-rhscl-7-rpms' is enabled for this system.
>subscription-manager repos --enable rhel-7-workstation-devtools-rpms
Repository 'rhel-7-workstation-devtools-rpms' is enabled for this system.

You can now install freely:

>yum update -y 
>yum install -y devtoolset-7-gcc-c++

Quite why they make you jump through so many hoops is explained by RedHat as follows:

-If you have a 2013 RHEL SKU with Standard or Premium service level, there should be no action on your part, and your subscription should have full access to RHSCL.

-If you have a 2010 or older RHEL SKU with Standard or Premium service level, you should contact Red Hat Customer Service to request the RHSCL SKU (and all related content) added to your account.

NOTE: Developer Toolset is now included as part of Red Hat Software Collections. This change was made on May 29, 2014.

As you had Red Hat Enterprise Linux Workstation, Standard subscription which is older RHEL SKU with Standard service level, you had to initiate a special request for it

I've confirmed that you can upgrade gcc from the default version 4.8 on centOS 7.

First, we need to install "Software Collections" in order to access some of the community packages including gcc v7

• sudo yum install -y centos-release-scl

Next, we want to install a developer toolset. Depending on your needs, you may want a different devtoolset. Here I'm targeting 7:

• sudo yum install -y devtoolset-7

Finally, you'll want to change over to gcc 7 as your default, launch a new shell session with the scl tool:

• scl enable devtoolset-7 bash

Now devtools-8 is available and it's possible to use it by following commands:

yum install centos-release-scl
yum install devtoolset-8-gcc devtoolset-8-gcc-c++
scl enable devtoolset-8 -- bash

It's explained in a little more detail in this answer on StackOverflow: How to install GCC/G++ 8 on CentOS. And of course, if you need GFortran, you can install it in the same way - the package is devtoolset-8-gcc-gfortran.

To install the full tools-set including gfortran on centos 7:

yum install centos-release-scl
yum install devtoolset-8
scl enable devtoolset-8 -- bash

enable the tools:

source /opt/rh/devtoolset-8/enable 

you may wish to put the command above in .bash_profile

ref: https://unix.stackexchange.com/questions/477360/centos-7-gcc-8-installation

Install it by:

sudo yum install centos-release-scl
sudo yum install devtoolset-4

The first command installs and enables Software Collections Repository on your CentOS machine. That repository provides the devtoolset package.

I used following steps on CentOS-7 and they worked fine for me:

yum install centos-release-scl
yum install devtoolset-10
scl enable devtoolset-10 bash

I used same flags which you used:

g++ -std=c++2a -O0 -g3 -Wall -c -fmessage-length=0 -MMD -MP -MF"src/dummy2.d" -MT"src/dummy2.o" -o "src/dummy2.o" "../src/dummy2.cpp"

What I have so far:

cat Dockerfile

FROM centos:7 AS env

RUN yum update -y
RUN yum install -y centos-release-scl
RUN yum install -y devtoolset-9

RUN echo "source /opt/rh/devtoolset-9/enable" >> /etc/bashrc
SHELL ["/bin/bash", "--login", "-c"]
RUN gcc --version

So you must:

1. Add the source stuff in a bashrc
   note: On Centos it's /etc/bashrc while on ubuntu it's /etc/bash.bashrc

2. Update the docker default shell to be bash AND to "load" the bashrc using --login

Output

docker build .
Sending build context to Docker daemon  4.096kB
Step 1/32 : FROM centos:7 AS env
 ---> 8652b9f0cb4c
Step 2/32 : RUN yum update -y
 ---> Using cache
 ---> a2bb269cd8dc
Step 3/32 : RUN yum install -y centos-release-scl
 ---> Using cache
 ---> 1184e26c71cf
Step 4/32 : RUN yum install -y devtoolset-9
 ---> Using cache
 ---> e678665d2a4e
Step 5/32 : RUN echo "source /opt/rh/devtoolset-9/enable" >> /etc/bashrc
 ---> Using cache
 ---> fe1745d4ca87
Step 6/32 : SHELL ["/bin/bash", "--login", "-c"]
 ---> Running in 2dd7955f4487
Removing intermediate container 2dd7955f4487
 ---> 3cf4835bf680
Step 7/32 : RUN gcc --version
 ---> Running in b5de3266d607
gcc (GCC) 9.3.1 20200408 (Red Hat 9.3.1-2)
Copyright (C) 2019 Free Software Foundation, Inc.
This is free software; see the source for copying conditions.  There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
 ...

What won't work

Test 1

RUN scl enable devtoolset-9 bash
RUN gcc --version | head -1

each RUN is a new shell so the sub-bash is lost on the second line.

Test 2

RUN source /opt/rh/devtoolset-9/enable && gcc --version | head -1
RUN gcc --version | head -1

Here again the source is only for the first RUN shell command but will be lost...

Test 3

This may work but with potential unexpected behaviour

ENV PATH=/opt/rh/devtoolset-9/root/bin:$PATH
RUN gcc --version | head -1

here we only "fix" the PATH variable but if you look at the /opt/rh/devtoolset-9/enable script there is so more to do than only updating the PATH...