Random postfixing container IDs

[zjrgov]

Story

As a devtools engineer,
I would like to have random postfixes on container ids,
So that I can worry less about their names being easy to guess.

Related to this PR #83 discussion.

Current ID string is combined as follows:

cfg.ContainerID = fmt.Sprintf(
	"glrw-r%v-p%v-c%v-j%v",
	cfg.RunnerID,
	cfg.ProjectID,
	cfg.ConcurrentProjectID,
	cfg.JobID,
)

Container ID should be at most 63 characters.

Solution

### Tasks
- [ ] Drop the runner ID segment because workers in a space will always have the same one.
- [ ] Add a random postfix to the ID.

Acceptance Criteria

• Generated container IDs are not too long.
• IDs are postfixed with some random string.