Add comment about gdb usage in docker

CAP_PTRACE is needed for ASLR disable to work, which gdb needs

build-envs/README

@@ -25,6 +25,13 @@ Running things by hand
 
   $ docker pull registry.gitlab.com/cerl/maestro/maestro-core/buildenv/debian
   $ docker run --ulimit memlock=133000:133000 -i -t registry.gitlab.com/cerl/maestro/maestro-core/buildenv/debian /bin/bash 
+
+  # but since you're reading this, you may be interested in debugging. In that
+  # case, to permit gdb to work inside the docker container you might want to
+  # use
+  #  docker run --cap-add=SYS_PTRACE --ulimit memlock=133000:133000 -i -t registry.gitlab.com/cerl/maestro/maestro-core/buildenv/debian /bin/bash
+  # instead
+
   # now you have a shell in the docker container that is currently used on gitlab for CI
   $ mkdir /builds
   $ cd /builds