Add support for container kernel capabilities (#716)

This patch adds two new command-line flags to specify one or
more kernel capabilities to add or remove from the workflow
containers.

The command-line flag `--container-cap-add` allows for adding
specific capabilities on the workflow containers; where as,

The command-line flag `--container-cap-drop` allows for removing
specific capabilities on the workflow containers.

This was developed to specifically be able to add `SYS_PTRACE`
to a workflow I maintain. It involves using this capability to
monitor a make build, to then build a compilation database.

Signed-off-by: Joseph Benden <joe@benden.us>

pkg/runner/run_context.go

@@ -148,7 +148,7 @@ func (rc *RunContext) startJobContainer() common.Executor {
 		return common.NewPipelineExecutor(
 			rc.JobContainer.Pull(rc.Config.ForcePull),
 			rc.stopJobContainer(),
-			rc.JobContainer.Create(),
+			rc.JobContainer.Create(rc.Config.ContainerCapAdd, rc.Config.ContainerCapDrop),
 			rc.JobContainer.Start(false),
 			rc.JobContainer.UpdateFromEnv("/etc/environment", &rc.Env),
 			rc.JobContainer.Exec([]string{"mkdir", "-m", "0777", "-p", ActPath}, rc.Env, "root"),