Add note on architecture differences when building containers: ARM v x86

[aturner-epcc]

As the number of attendees using newer Mac models with ARM-based processors increases, the problem of containers built on Macs being incompatible with other (x86-based) systems (and vice-versa) is going to become more prevalent.

We should add a note/callout to the building containers episode that briefly describes the issue, illustrates the --platform option to docker image build and describes good practice in make both x86 and ARM versions of container images available. (I know you can create multi-architecture containers but I think that, unless the image is tiny, these will be too large and unwieldy for general use.)

I am happy to put in a PR for this.