Allow using an arbitrary Docker image instead of forcing crates-build-env

[pietroalbini]

While Rustwide technically already allows to run a build with an arbitrary container image, it forces you to use (a fork of) crates-build-env as it integrated with it (through the MAP_USER_ID environment variable) to drop privileges inside the container.

This PR lifts that restriction by using Docker's --user flag to drop privileges, instead of the entrypoint in crates-build-env.

[jyn514]

2020-07-03T12:57:36.9284918Z [INFO  rustwide::cmd] running `Command { std: "docker" "create" "-v" "/home/runner/work/rustwide/rustwide/.workspaces/integration/builds/out-of-memory/target:/opt/rustwide/target:rw,Z" "-v" "/home/runner/work/rustwide/rustwide/.workspaces/integration/builds/out-of-memory/source:/opt/rustwide/workdir:ro,Z" "-v" "/home/runner/work/rustwide/rustwide/.workspaces/integration/cargo-home:/opt/rustwide/cargo-home:ro,Z" "-v" "/home/runner/work/rustwide/rustwide/.workspaces/integration/rustup-home:/opt/rustwide/rustup-home:ro,Z" "-e" "SOURCE_DIR=/opt/rustwide/workdir" "-e" "CARGO_TARGET_DIR=/opt/rustwide/target" "-e" "CARGO_HOME=/opt/rustwide/cargo-home" "-e" "RUSTUP_HOME=/opt/rustwide/rustup-home" "-w" "/opt/rustwide/workdir" "-m" "536870912" "--user" "1001:116" "--network" "none" "rustops/crates-build-env@sha256:819119df93c0f5eca3f29186f14981ef29945a311854d7222af07488600a2584" "/opt/rustwide/cargo-home/bin/cargo" "+stable" "run", kill_on_drop: false }`
2020-07-03T12:57:36.9285260Z [INFO  rustwide::cmd] [stdout] 6ed676cd022fb006c0792904bc334784e6ff9d87a9a45ce4ee0f6f5d84b84737
2020-07-03T12:57:36.9285852Z [INFO  rustwide::cmd] running `Command { std: "docker" "start" "-a" "6ed676cd022fb006c0792904bc334784e6ff9d87a9a45ce4ee0f6f5d84b84737", kill_on_drop: false }`
2020-07-03T12:57:36.9286362Z [INFO  rustwide::cmd] [stderr] /opt/crates-build-env/entrypoint.sh: line 7: /etc/hosts: Permission denied
2020-07-03T12:57:36.9286831Z [INFO  rustwide::cmd] [stderr]    Compiling out-of-memory v0.1.0 (/opt/rustwide/workdir)
2020-07-03T12:57:36.9286991Z [INFO  rustwide::cmd] [stderr]     Finished dev [unoptimized + debuginfo] target(s) in 0.28s
2020-07-03T12:57:36.9287457Z [INFO  rustwide::cmd] [stderr]      Running `/opt/rustwide/target/debug/out-of-memory`
2020-07-03T12:57:36.9287633Z [INFO  rustwide::cmd] running `Command { std: "docker" "inspect" "6ed676cd022fb006c0792904bc334784e6ff9d87a9a45ce4ee0f6f5d84b84737", kill_on_drop: false }`
2020-07-03T12:57:36.9288497Z [INFO  rustwide::cmd] running `Command { std: "docker" "rm" "-f" "6ed676cd022fb006c0792904bc334784e6ff9d87a9a45ce4ee0f6f5d84b84737", kill_on_drop: false }`
2020-07-03T12:57:36.9288677Z [INFO  rustwide::cmd] [stdout] 6ed676cd022fb006c0792904bc334784e6ff9d87a9a45ce4ee0f6f5d84b84737
2020-07-03T12:57:36.9289477Z thread 'buildtest::test_sandbox_oom' panicked at 'didn't get the error CommandError::SandboxOOM', tests/buildtest/mod.rs:79:13

Not sure if the /etc/hosts failure is relevant. The user is being passed, as is the memory limit ... not sure what's going wrong.

[pietroalbini]

🎉