GUACAMOLE-2264: Set CMAKE_BUILD_TYPE=Release for FreeRDP production builds.

[escra]

Summary

The official Dockerfile builds FreeRDP without specifying CMAKE_BUILD_TYPE.
CMake defaults to an empty build type, which in FreeRDP's case enables
WINPR_ASSERT runtime assertions (equivalent to WITH_VERBOSE_WINPR_ASSERT=ON).

When FreeRDP encounters any unexpected condition at runtime, WINPR_ASSERT
calls abort(), which sends SIGABRT to the guacd child process. The child
process terminates silently -- no error message is logged by guacd because
the process is killed by the signal before it can log anything.

Observable symptoms

FreeRDP logs this warning at connection startup:
`

---

This build is using [runtime-check] build options:

• 'WITH_VERBOSE_WINPR_ASSERT=ON'

[runtime-check] build options might slow down the application

---

`

Then, under certain conditions (e.g., unexpected server responses, edge cases
in NLA negotiation), the child process disappears:
guacd[1]: INFO: Connection "..." removed. guacd[1]: DEBUG: Unable to request termination of client process: No such process

Impact

• Random, silent RDP connection failures
• No error diagnostics -- the crash reason is not logged
• More likely with FreeRDP 3.x (significantly more assertions than 2.x)
• Particularly problematic with Windows servers that have non-standard
  configurations (NLA edge cases, unusual certificate setups)

Fix

Add -DCMAKE_BUILD_TYPE=Release to FREERDP_OPTS in the Dockerfile. This:

• Disables WINPR_ASSERT (assertions become no-ops in Release builds)
• Enables compiler optimizations (-O2 / -O3)
• Strips debug symbols (smaller binary)
• Follows standard practice for production builds

Risk

Very low. CMAKE_BUILD_TYPE=Release is the standard CMake configuration
for production builds. FreeRDP's own CI and all Linux distribution packages use
Release builds. The assertions are developer aids, not production safety nets --
they catch "should never happen" conditions by crashing, which is worse than
graceful error handling in production.

Test plan

• Build Docker image with the change and verify no [runtime-check] warning
• Verify RDP connections to Windows servers with NLA are stable
• Verify no performance regression (Release mode is actually faster)
• Compare image size (expect slight reduction from stripped debug info)