Install net-tools package (#79)

To debug the flaky network-related tests in `rippled`, we would like to
use `netstat` to check network statistics. However, none of the CI
images contain that application. This PR installs `net-tools`, which
includes `netstat`.

Author: bthomee

docker/debian/Dockerfile

@@ -50,6 +50,7 @@ pkgs+=(gpg-agent)       # Dependency for tools requiring signing or encrypting/d
 pkgs+=(jq)              # JSON manipulation.
 pkgs+=(libatomic1)      # Required by mold linker.
 pkgs+=(libc6-dev)       # Required build tool.
+pkgs+=(net-tools)       # Dependency for debugging network issues.
 pkgs+=(ninja-build)     # Required build tool.
 pkgs+=(python3-venv)    # Python environment management tool.
 pkgs+=(python3-pip)     # Package manager for Python applications.

docker/rhel/Dockerfile

@@ -26,6 +26,7 @@ pkgs+=(gnupg2)            # Dependency for tools requiring signing or encrypting
 pkgs+=(jq)                # JSON manipulation.
 pkgs+=(libatomic)         # Required by mold linker.
 pkgs+=(libstdc++-static)  # Required to statically link libraries into rippled.
+pkgs+=(net-tools)         # Dependency for debugging network issues.
 pkgs+=(ninja-build)       # Required build tool.
 pkgs+=(perl-Digest-SHA)   # Required to compile dependencies.
 pkgs+=(perl-IPC-Cmd)      # Required to compile dependencies.

docker/ubuntu/Dockerfile

@@ -36,6 +36,7 @@ pkgs+=(gpg-agent)       # Dependency for tools requiring signing or encrypting/d
 pkgs+=(jq)              # JSON manipulation.
 pkgs+=(libatomic1)      # Required by mold linker.
 pkgs+=(libc6-dev)       # Required build tool.
+pkgs+=(net-tools)       # Dependency for debugging network issues.
 pkgs+=(ninja-build)     # Required build tool.
 pkgs+=(python3-venv)    # Python environment management tool.
 pkgs+=(python3-pip)     # Package manager for Python applications.