docker: force amd64 arch

Building docker on an arm64 machine (our new runners) uses arm64 images, but AWS lightsail apparently only supports amd64, resulting on the following error when deploying:

exec /usr/local/bin/docker-entrypoint.sh: exec format error

This MR adds --platform=linux/amd64 to the Dockerfile of our image.

More info:

• https://github.com/docker/compose/issues/10185
• https://stackoverflow.com/questions/73398714/docker-fails-when-building-on-m1-macs-exec-usr-local-bin-docker-entrypoint-sh