fix: container optional python dependencies

See snyk/snyk-docker-plugin#614

Fixing a bug in snyk-docker-plugin so that optional dependencies are
properly connected in the dep-graph.

This could mean that 'snyk container monitor' commands that previously
timed out or errored may now start working. The fix reduces
unnecessary (optional) paths in the dep-graph and so reduce the work
Snyk needs to do when scanning for vulnerabilities.

package.json

@@ -116,7 +116,7 @@
     "semver": "^6.0.0",
     "snyk-config": "^5.0.0",
     "snyk-cpp-plugin": "2.24.0",
-    "snyk-docker-plugin": "6.13.11",
+    "snyk-docker-plugin": "6.13.15",
     "snyk-go-plugin": "1.23.0",
     "snyk-gradle-plugin": "4.6.0",
     "snyk-module": "3.1.0",