Fix potential unexpected current directory inclusion in Docker OAP classpath

[weixiang1862]

Fix potential unexpected current directory inclusion in Docker OAP classpath

• Add a unit test to verify that the fix works.
• Explain briefly why the bug exists and how to fix it.

Problem

In docker/oap/docker-entrypoint.sh, the classpath is constructed as:

CLASSPATH="config:$CLASSPATH"

When the $CLASSPATH environment variable is unset or empty (which is the common case in container environments), this produces a classpath string ending with a trailing colon:

CLASSPATH="config:"

An empty path element in a colon-separated classpath (e.g., config: or dir1::dir2) is interpreted by the JVM as the current working directory (.).
This means the trailing colon in config: implicitly adds the OAP container's working directory (/skywalking) to the classpath.

Fix

Added a null check before concatenating the existing $CLASSPATH, aligning the Docker startup script with the existing dist-material/bin/ scripts:

if [ -z "$CLASSPATH" ]; then
  CLASSPATH="config"
else
  CLASSPATH="config:$CLASSPATH"
fi

• If this pull request closes/resolves/fixes an existing issue, replace the issue number. Closes #.
• Update the CHANGES log.