BROKEN: rewrite dockerfile for improved caching

- build stage depends on an image which already has Rust present
- perform all stage-initalization steps before copying any data,
  preserving maximum caching
- build all deps in a separate cached stage so changes only require
  rebuilding the affected code
- build each module independently in reverse topo order so unaffected
  build stages can in principle retain their cache

Unfortunately, the steps above still don't work: despite the presence
of log lines like

```
 ---> 104821d12748
Step 32/54 : RUN cargo build --$PROFILE -p polkadot-primitives
 ---> Running in a78deef9950c
    Finished release [optimized] target(s) in 0.97s
Removing intermediate container a78deef9950c
```

we still end up with errors like

```
Step 37/54 : RUN cargo build --$PROFILE -p polkadot-runtime-common
 ---> Running in 9f65497b7c6d
   Compiling polkadot-runtime-common v0.7.17 (/polkadot/runtime/common)
error[E0432]: unresolved imports `primitives::Hash`, `primitives::parachain`
  --> runtime/common/src/attestations.rs:26:18
   |
26 | use primitives::{Hash, parachain::{AttestedCandidate, CandidateReceipt, Id as ParaId}};
   |                  ^^^^  ^^^^^^^^^ could not find `parachain` in `primitives`
   |                  |
   |                  no `Hash` in the root

```

This optimization has now consumed more dev time than it's worth. I'm
preserving it in git just in case, but it's time to simplify.