Run docker build less often

[jbr]

Currently, the slowest GitHub actions are the docker builds. The only ways the docker builds would fail are:

• a new top level file or directory that is COPY-ed is added or removed
• npm ci or npm run build fail
• cargo build fails
• the Dockerfile includes changes that need iteration

npm ci/run build and cargo build are covered directly by other GitHub actions

In order to mitigate against merging breaking changes to main, we could run the docker builds IFF the list of top level files has changed, or if the Dockerfile has changed