fix(docker): ensure PLATFORM is forwarded correctly in Docker build

This patch resolves an inconsistency where specifying PLATFORM=...on
the host was ignored during Docker-based builds.

Changes:
- Forward "PLATFORM" variable to container's environment in
  run_in_container.sh
- Update Makefile to pass PLATFORM to the nested make invocation
  inside Docker.
- Fix a SyntaxWarning in check_platform_exists.py by using a raw
  string for the regex.

This ensures platform-specific builds behave identically inside and
outside Docker.

Change-Id: Iad3b0cc1625c45e62d203adf64f6bf8a2f07f173
Signed-off-by: Jayanth Dodderi Chidanand <jayanthdodderi.chidanand@arm.com>
diff --git a/build/check_platform_exists.py b/build/check_platform_exists.py
index 2675d49..8441936 100755
--- a/build/check_platform_exists.py
+++ b/build/check_platform_exists.py
@@ -25,7 +25,7 @@
     project = sys.argv[1]
 
     platforms = []
-    reg = re.compile('aarch64_toolchains\("(\w*)')
+    reg = re.compile(r'aarch64_toolchains\("(\w*)')
     with open("project/" + project + "/BUILD.gn") as project_file:
         for line in project_file:
             platforms += reg.findall(line)