Add trn2.48xlarge, set workdir to /home/ubuntu in neuron container image

[mselim00]

Issue #, if available:

• Add trn2.48xlarge
• Set workdir to /home/ubuntu in container image for neuron. Alleviates in issue where programs that try to write to the working directory are hit with irreconcilable permission denied errors due to the attempt to write to the root directory.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[mattcjo]

Not sure it really matters, but everything under the eks directory is no longer supported AFAIK (@cartermckinnon please confirm/deny) . Otherwise LGTM.

[cartermckinnon]

The stuff under eks isn't used in our kubetest2 setup, no