Multi arch Dockerfile and CI

Dockerfile changes

Use Debian based :1-bullseye-slim Rust image for cross compilation instead of :1-alpine3.13, because:
- it's 20MB smaller
- Alpine 3.13 does not work well on 32 bit systems with an older libseccomp like on Raspberry Pis
- it does not require to install musl cross building tools for the build platform so it's faster ⚡
- it's available on more platforms than just amd64 and arm64 so users could build the image on their raspberry pis if they want to
Cross compile on native build platform so it's faster than Docker QEMU emulation ⚡
Compile static binaries by bundling musl in the binary (as before, note that this uses +3MB)
Set variables correctly for all Docker supported platforms
- ppc64le, s390x and riscv64 do not support the Rust standard lib so it will fail for these
- Persist target platform variables through files in /tmp/
Define linkers to use for each musl target in .cargo/config
Cache dependencies properly for target platform (so it's fast ⚡ with 1s rebuilds 🎉)
Scratch stage to test the binary is static and built correctly for the target platform
Remove useless (and long) cargo install instruction ⚡
Move statically compiled binary to /tmp to have a shorter COPY instruction for readability

Github Actions workflows were changed to build for 386, arm64, armv6 and armv7 on top of amd64.

See #48 for an updated list of things left to do.

@MindFlavor feel free to ask questions/suggestions and approve the PR so we can finally have cross built Docker images 🎉