Make Rust toolchain available during package building

[legoktm]

Status

Ready for review, but depends on #1817 to be merged first.

Description

So far we've been using an ad-hoc image in which the necessary
dependencies are all downloaded at build time, which is nice for tooling
simplicity but adds up the more things we need to download each time.

This adopts the same system used in the SecureDrop server build process.
The tooling builds an image with some dependencies and installs the
minimal Rust toolchain in an image. The image_prop and dep-check scripts
verify all the dependencies are up to date, and rebuild the image if
not.

Test Plan

• Visual review
• Build a Rust-based package in Implement proxy v2 architecture, in Rust #1718

[cfm]

Rebased (actually, cherry-picked, to avoid having to resolve the same conflicts) from main after #1817.

[cfm]

• Build a Rust-based package in Implement proxy v2 architecture, in Rust #1718

That's a little sneaky, since it means reviewing on rust-packaging and testing on proxy-rusting. ;-) But I can confirm that the container builds on rust-packaging and builds securedrop-proxy on proxy-rusting.

However:

user@sd:~/securedrop-client/build$ dpkg-deb -c ./securedrop-proxy_0.9.0+bullseye_all.deb | grep "securedrop-proxy" | grep "usr/bin"
lrwxrwxrwx root/root         0 2023-03-16 13:29 ./opt/venvs/securedrop-proxy/bin/python -> /usr/bin/python3
lrwxrwxrwx root/root         0 2023-03-16 13:29 ./usr/bin/sd-proxy -> /opt/venvs/securedrop-proxy/bin/sd-proxy

Am I right that no change in the final package actually output is expected here?

[cfm]

My goof. securedrop-proxy is built in an amd64-architecture package, which therefore won't overwrite the old all package:

user@sd:~/securedrop-client/build$ ls -al securedrop-proxy*.deb
-rw-r--r-- 1 root root 1232512 Feb 15 11:56 securedrop-proxy_0.9.0+bullseye_all.deb
-rw-r--r-- 1 root root 1069724 Feb 15 12:07 securedrop-proxy_0.9.0+bullseye_amd64.deb
-rw-r--r-- 1 root root 1093408 Feb 15 12:07 securedrop-proxy-dbgsym_0.9.0+bullseye_amd64.deb
user@sd:~/securedrop-client/build$ dpkg-deb -c ./securedrop-proxy_0.9.0+bullseye_amd64.deb | grep "securedrop-proxy" | grep "usr/bin"
-rwxr-xr-x root/root   3485976 2023-03-16 13:29 ./usr/bin/securedrop-proxy

Full steam ahead.