fix(up): add Service.containerName helper for explicit container_name

[YvanY0]

The bug

When a compose file sets an explicit container_name, the container starts correctly — container run --name reads the value. But ComposeUp has three other places that look up the running container by name (waitUntilServiceIsRunning, getIPForRunningService, stopOldStuff), and all three hard-code "\(projectName)-\(serviceName)" instead of reading container_name.

For example, given:

services:
  hermes:
    container_name: hermes

Run from a directory named hermes (no name: field), the container starts as hermes but the post-start poll waits for hermes-hermes and times out:

Error Domain=ContainerWait Code=1 "Timed out waiting for container 'hermes-hermes' to be running."

Issue #38 reported the same bug on the down side; PR #39 fixed ComposeDown.stopOldStuff but ComposeUp was not addressed.

The fix

Extract Service.containerName(projectName:serviceName:) — a single method that encapsulates the rule (prefer explicit container_name, fall back to "\(projectName)-\(serviceName)") — and route all five call sites through it:

• 4 call sites in ComposeUp (configService, waitUntilServiceIsRunning, getIPForRunningService, stopOldStuff)
• stopOldStuff in ComposeDown (refactored to use the same helper, replacing the inline if/else from PR Handle compose down with container_name case #39)

When container_name is not set the helper returns the same "\(projectName)-\(serviceName)" string as before; existing compose files without explicit container names are unaffected.

Verification

Tested locally against the compose file above:

Before: container-compose up -d reports Timed out waiting for container 'hermes-hermes' to be running.

After: the container starts and polls successfully with no timeout; container ls -a shows it in running state.

Tests

• Static: ServiceNamingTests — two unit tests for the pure containerName helper (explicit name wins, default pattern when unset).
• Dynamic: testUpWithExplicitContainerName — exercises ComposeUp.run() end-to-end with an explicit container_name (mirrors the pattern in PR feat: Providing tests for ComposeDown #50's testUpAndDownContainerName).

Fixes #38