https://getsol.us/solbuild/{main,unstable}-x86_64.img.xz. Please be responsible and don't spam with downloads. If you have solbuild initialized you can just copy from /var/lib/solbuild/images/ instead to save the download. The images themselves don't get updated too often as they are mostly used for solbuild and we can update them locally once they are initialized. The images contain system.base and system.devel components.
Or spin them yourself (this is untested by me but should work)
mkdir -p solusrootfs/var/cache/eopkg/packages
mkdir -p solusrootfs/var/cache/eopkg/archives
# Avoid redownloading unnecessarily
mount --bind /var/cache/eopkg/packages solusrootfs/var/cache/eopkg/packages
# Add repo (shannon or unstable)
eopkg add-repo Solus https://mirrors.rit.edu/solus/packages/shannon/eopkg-index.xml.xz -D solusrootfs
# Install stuff (not sure if you'll need ignore safety and ignore-comar)
eopkg it --ignore-comar -D solusrootfs --ignore-safety baselayout -y
eopkg it --ignore-comar -D solusrootfs --ignore-safety -c system.base -y
eopkg it --ignore-comar -D solusrootfs --ignore-safety -c system.devel -y
eopkg it --ignore-comar -D solusrootfs --ignore-safety another other packages you want -y
umount solusrootfs/var/cache/eopkg/packages
Then tar that up as you need it.
edit: Just to reiterate solus containers are not supported in any way.
