Add gdb to Docker test containers

[zhengyu123]

What does this PR do?:

• Install gdb in Alpine (musl) base image
• Add --cap-add=SYS_PTRACE and --init/--ulimit flags to shell mode so gdb can ptrace processes inside the container

Motivation:
Add ability to debug crashes encountered during the test runs.

Additional Notes:

How to test the change?:

• Run ./utils/run-docker-tests.sh --libc=musl --jdk=11 --shell --mount
• Inside docker shell, run gdb
• gdb should execute

For Datadog employees:

• If this PR touches code that signs or publishes builds or packages, or handles
  credentials of any kind, I've requested a review from @DataDog/security-design-and-guidance.
• This PR doesn't touch any of that.
• JIRA: [JIRA-XXXX]

Unsure? Have a question? Request a review!

[dd-octo-sts]

CI Test Results

Run: #22366891154 | Commit: 7115d81 | Duration: 11m 24s (longest job)

✅ All 40 test jobs passed

Status Overview

JDK	glibc-aarch64/debug	glibc-amd64/debug	musl-aarch64/debug	musl-amd64/debug
8	-	✅	-	-
8-ibm	-	✅	-	-
8-j9	✅	✅	-	-
8-librca	-	-	✅	✅
8-orcl	-	✅	-	-
8-zing	✅	✅	-	-
11	-	✅	-	-
11-j9	✅	✅	-	-
11-librca	-	-	✅	✅
11-zing	✅	✅	-	-
17	✅	✅	-	-
17-graal	✅	✅	-	-
17-j9	✅	✅	-	-
17-librca	-	-	✅	✅
17-zing	✅	✅	-	-
21	✅	✅	-	-
21-graal	✅	✅	-	-
21-librca	-	-	✅	✅
21-zing	✅	✅	-	-
25	✅	✅	-	-
25-graal	✅	✅	-	-
25-librca	-	-	✅	✅

Legend: ✅ passed | ❌ failed | ⚪ skipped | 🚫 cancelled

Summary: Total: 40 | Passed: 40 | Failed: 0

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Updated: 2026-02-24 19:50:36 UTC