fix: reworked container architecture (#619)

- Don't set architecture, let Docker host decide it's own platform,
  remove `runtime` dependency and don't show default in `--help`
- Remove most tests, we need to check only once if it works on
  different platform
- Rename `DeleteImage` to `RemoveImage` to conform to existing
  function in `docker` cli, added options to specify `force` and
  `pruneChildren`

pkg/runner/run_context.go

@@ -96,10 +96,6 @@ func (rc *RunContext) startJobContainer() common.Executor {
 			binds = append(binds, fmt.Sprintf("%s:%s%s", rc.Config.Workdir, rc.Config.Workdir, bindModifiers))
 		}
 
-		if rc.Config.ContainerArchitecture == "" {
-			rc.Config.ContainerArchitecture = fmt.Sprintf("%s/%s", "linux", runtime.GOARCH)
-		}
-
 		rc.JobContainer = container.NewContainer(&container.NewContainerInput{
 			Cmd:        nil,
 			Entrypoint: []string{"/usr/bin/tail", "-f", "/dev/null"},