podman build fails with "cp: cannot access '/root/files/mono-6.12.0.147': Permission denied"

Trying to run
```sh
./build.sh 3.x mono-6.12.0.147
```

fails for me in trying to build this container:
```
STEP 1/4: FROM godot-fedora:3.x-mono-6.12.0.147
STEP 2/4: ARG mono_version
--> 29b0f1a0983
STEP 3/4: RUN if [ -z "${mono_version}" ]; then echo -e "\n\nargument mono-version is mandatory!\n\n"; exit 1; fi && dnf -y install --setopt=install_weak_deps=False autoconf automake cmake gcc gcc-c++ gettext libtool perl python-unversioned-command && cp -a /root/files/${mono_version} /root && cd /root/${mono_version} && NOCONFIGURE=1 ./autogen.sh && ./configure --prefix=/usr --sysconfdir=/etc --localstatedir=/var/lib/mono --disable-boehm --host=x86_64-linux-gnu && make -j && make install && cd /root && rm -rf /root/${mono_version} && cert-sync /etc/pki/tls/certs/ca-bundle.crt && rpm -ivh --nodeps https://download.mono-project.com/repo/centos8-preview/m/msbuild/msbuild-16.10.1+xamarinxplat.2021.05.26.14.00-0.xamarin.7.epel8.noarch.rpm https://download.mono-project.com/repo/centos8-preview/m/msbuild/msbuild-sdkresolver-16.10.1+xamarinxplat.2021.05.26.14.00-0.xamarin.7.epel8.noarch.rpm https://download.mono-project.com/repo/centos8-preview/m/msbuild-libhostfxr/msbuild-libhostfxr-3.0.0.2019.04.16.02.13-0.xamarin.4.epel8.x86_64.rpm https://download.mono-project.com/repo/centos8-preview/n/nuget/nuget-5.6.0.6489.bin-0.xamarin.1.epel8.noarch.rpm
```

The first few containers seem to build fine:
```
COMMIT godot-fedora:3.x-mono-6.12.0.147
COMMIT godot-export:3.x-mono-6.12.0.147
```

This one failing container then in turn seems to cause all the rest of the containers to fail to build:
```
STEP 1/4: FROM godot-mono:3.x-mono-6.12.0.147
Resolving "godot-mono" using unqualified-search registries (/etc/containers/registries.conf)
Trying to pull registry.fedoraproject.org/godot-mono:3.x-mono-6.12.0.147...
Trying to pull registry.access.redhat.com/godot-mono:3.x-mono-6.12.0.147...
Trying to pull docker.io/library/godot-mono:3.x-mono-6.12.0.147...
Trying to pull quay.io/godot-mono:3.x-mono-6.12.0.147...
Error: error creating build container: 4 errors occurred while pulling:
* initializing source docker://registry.fedoraproject.org/godot-mono:3.x-mono-6.12.0.147: reading manifest 3.x-mono-6.12.0.147 in registry.fedoraproject.org/godot-mono: manifest unknown: manifest unknown
* initializing source docker://registry.access.redhat.com/godot-mono:3.x-mono-6.12.0.147: reading manifest 3.x-mono-6.12.0.147 in registry.access.redhat.com/godot-mono: name unknown: Repo not found
* initializing source docker://godot-mono:3.x-mono-6.12.0.147: reading manifest 3.x-mono-6.12.0.147 in docker.io/library/godot-mono: errors:
denied: requested access to the resource is denied
unauthorized: authentication required
```

Entire build log:
[log.txt](https://github.com/godotengine/build-containers/files/7486481/log.txt)

This is on Fedora 34 host, and just freshly pulled in Fedora 34 docker image.

Edit: running with selinux disabled temporarily (`sudo setenforce 0`) seems to allow the containers to be built correctly.
Fedora has selinux on by default (I think, at least I've never voluntarily followed a guide to enable it) so this repo should be updated to work with it, or explicit instructions should be included in the readme for disabling it.