Use builder pattern to reduce image sizes

Builds on top of #53 to use the builder pattern. Here, we do general work in a temporary builder stage and copy only the required files into the rust-optimizer and workspace-optimizer images. Anything that doesn't get copied into the final images is thrown away. This reduces image sizes by ~10% to 900MB. Note that rust:1.54.0-alpine is 786MB before we add any wasm specific stuff, so only adding ~100MB is quite good.

❯ docker image ls
REPOSITORY                     TAG             IMAGE ID       CREATED         SIZE
cosmwasm/rust-optimizer        0.12.1          4f80eb3b2ad0   3 hours ago     902MB
cosmwasm/workspace-optimizer   0.12.1          b7bb3c9473e4   3 hours ago     924MB
cosmwasm/workspace-optimizer   0.12.0          b9f4ed28322e   4 hours ago     1.03GB
cosmwasm/rust-optimizer        0.12.0          517921e0c25b   4 hours ago     1.06GB

The way the Dockerfiles were written before meant that files that were removed in separate RUN commands were still present in other layers, e.g.

https://github.com/CosmWasm/rust-optimizer/blob/5a519d85ec6a69fe46b7864607b0092267ade209/Dockerfile#L20-L23

The 0.12.0 and 0.12.1 containers produce identical wasm files:

❯ docker run --rm -v "$(pwd)":/code \
  --mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
  --mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
  cosmwasm/rust-optimizer:0.12.0
...
❯ cat artifacts/checksums.txt
a25ad28b91754a99cfeadcb8f7f6f0570a118378cbd284938620089b4ada5268  terraswap_factory.wasm
f0a4828cd0df981d451e8544cc4d5f530b5e47c6c487856c0e9ced907da11d0e  terraswap_pair.wasm
d68f8b74597e9f374b48aad44ea7746203de4eee3b7cb9f9dde605ca43a21aa5  terraswap_router.wasm
86c1ed459c9030844adbbbae1793e05371672547ce387b33feb2c78100a796dd  terraswap_token.wasm
❯ docker run --rm -v "$(pwd)":/code \
  --mount type=volume,source="$(basename "$(pwd)")_cache",target=/code/target \
  --mount type=volume,source=registry_cache,target=/usr/local/cargo/registry \
  cosmwasm/rust-optimizer:0.12.1-rc1
...
❯ cat artifacts/checksums.txt
a25ad28b91754a99cfeadcb8f7f6f0570a118378cbd284938620089b4ada5268  terraswap_factory.wasm
f0a4828cd0df981d451e8544cc4d5f530b5e47c6c487856c0e9ced907da11d0e  terraswap_pair.wasm
d68f8b74597e9f374b48aad44ea7746203de4eee3b7cb9f9dde605ca43a21aa5  terraswap_router.wasm
86c1ed459c9030844adbbbae1793e05371672547ce387b33feb2c78100a796dd  terraswap_token.wasm

CosmWasm / optimizer

Use builder pattern to reduce image sizes #54