How can I build a specific package in a workspace?

You can use the --package or -p flag to cargo build:

cargo build                # build packages listed in the default-members key
cargo build --all          # build all packages
cargo build --package foo  # build the package "foo"
cargo build -p foo         # ditto

The first answer not work with:

error: package ID specification `foo` matched no packages

The correct way is:

find out pkgid first:

$ cd server
$ cargo pkgid

file:///dw/path/to/server:4.0.0-SNAPSHOT

Goto workspace folder and run build

$ cd -
cargo build -p file:///dw/path/to/server:4.0.0-SNAPSHOT