Restore valid base image version

[rmshaffer]

Looks like #437 contained an inadvertent change to Dockerfile that broke the Docker image build.

e.g., from https://github.com/microsoft/Quantum/runs/1383424059:

Step 1/10 : FROM mcr.microsoft.com/quantum/iqsharp-base:0.13.201118141-beta
manifest for mcr.microsoft.com/quantum/iqsharp-base:0.13.201118141-beta not found: manifest unknown: manifest tagged by "0.13.201118141-beta" is not found
Error response from daemon: No such image: public/quantum/samples:77b68faa38df8a601ba3fcc1ffef3094be242cba

This change reverts back to use the previously-released version 0.13.20102604.

[cgranade]

That's my bad, sorry; I should have caught that beta Docker images aren't actually published. Thanks for catching this!

[rmshaffer]

That's my bad, sorry; I should have caught that beta Docker images aren't actually published. Thanks for catching this!

No worries at all - that's why we have CI to catch these things!