Update sbt-sourcegraph to 0.4.3 (sourcegraph#647)

External docker build (sourcegraph#637)

* Build docker container in a self-contained fashion

This is prompted by various issues around our handling of arguments and
how this clashes with -- and -D argument handling by both sbt-pack and
sbt-native-packager

Instead, we build scip-java entirely inside the docker container and
point `scip-java` command directly at the jar, to exclude any
possibility of weirdness introduced by launcher scripts.

Additionally, this allows anyone to build a docker container without
having JVM and SBT installed.

* Update coursier launcher version and remove unnecessary architecture
override

convert to public API (sourcegraph#618)

Update sbt-sourcegraph and re-enable SBT build tool tests (sourcegrap…

…h#636)

Remove coursier bootstrap and respect COURSIER_JVM_INDEX (sourcegraph…

…#631)

Allow picking up credentials from environment variables (sourcegraph#629

)

* Upgrade Coursier to respect COURSIER_CREDENTIALS string

* Make sure we respect COURSIER_REPOSITORIES

* Add tests for Coursier env variables with a local server