updated Dockerfile with current debian stable (bullseye)

[mfrey]

I ran into an issue using the latest version of rust docker environment, i.e.

# RUST_LOG=cargo=debug cargo update
DEBUG:cargo::update: executing; cmd=cargo-update; args=["cargo", "update"]
DEBUG:cargo::core::workspace: find_root - trying /home/Cargo.toml
DEBUG:cargo::core::workspace: find_root - trying /Cargo.toml
DEBUG:cargo::core::workspace: find_members - only me as a member
DEBUG:cargo::core::registry: load/missing  file:///home/rust
DEBUG:cargo::sources::config: loading: file:///home/rust
DEBUG:cargo::core::resolver: initial activation: files v0.1.0 (file:///home/rust)
DEBUG:cargo::core::registry: load/missing  registry https://github.com/rust-lang/crates.io-index
DEBUG:cargo::sources::config: loading: registry https://github.com/rust-lang/crates.io-index
    Updating registry `https://github.com/rust-lang/crates.io-index`
DEBUG:cargo: handle_cli_error; err=CliError { error: Some(ChainedError { error: failed to load source
for a dependency on `url`, cause: ChainedError { error: Unable to update registry 
https://github.com/rust-lang/crates.io-index, cause: ChainedError { error: failed to fetch 
`https://github.com/rust-lang/crates.io-index`, cause: Error { code: -17, klass: 16, message: 
"The SSL certificate is invalid" } } } }), unknown: false, exit_code: 101 }

I think it is related to an outdated ca-certificates package. Since debian jessie is quite old, I updated the debian version from jessie to bullseye. I also had to add gpg to the build dependencies and also had to add a different key server (I ran into connection timeout issues). Build performed via:

docker build -t rust-docker-dev-env --build-arg rust_version=1.54.0 . 

P.S.: Thanks for the initial work! Great job!