Pin images to bookworm and install openssl

[WillNilges]

Closes #315

The runtime image for the API needs libssl to work. It looks like a version skew between the build and runtime images was causing the wrong version of libssl to be available, and the container wouldn't run. So, this PR...

• Pins builder to rust:bookworm (latest at time of PR)
• Pins runtime to debian:bookworm-slim (latest at time of PR)
• Installs openssl package in runtime image to satisfy libssl dependency

Updating the base image to the latest Debian ought to be a discrete decision to make, instead of being at the whims and mercy of latest 🙃 .

[wetzel-desy]

this change should also solve #305 if the specific GLIBC version is not important for crab.fit