fix(Dockerfile): update checksum for aws cert bundle

[firestack]

The AWS RDS Cert bundle updated again and builds have been failing due to that.

Turns out, there is now a Dockerfile --checksum argument to the ADD command which both pulls something from the internet (like curl) but also can check the hash of the content at the same time.

This will also give us better logs when builds fail, and tell us what the new checksum is.
(Example log from when I initially added the ADD command, with a dummy checksum from the docs)

ERROR: failed to solve: digest mismatch sha256:390fdc813e2e58ec5a0def8ce6422b83d75032899167052ab981d8e1b3b14ff2: sha256:24454f830cdb571e2c4ad15481119c43b3cafd48dd869a9b2945d1036d1dc68d

Previously: #2746
Asana Ticket: https://app.asana.com/0/1148853526253420/1208798175282938/f

[firestack]

The part that was failing was the build part of the deploy, so even without the deploy succeeding (which I will keep trying) this is good to go @mbta/skate-developers

[hannahpurcell]

So sorry for my long delay, and thank you 🙏