added build arg for custom packages

.gitignore

@@ -0,0 +1 @@
+.env

Dockerfile

@@ -1 +0,0 @@
-touch /etc/s6-overlay/s6-rc.d/user/contents.d/init-custom-users

build/Dockerfile

@@ -0,0 +1,9 @@
+FROM lscr.io/linuxserver/rdesktop:ubuntu-xfce
+
+ARG ADDITIONAL_PACKAGES=""
+ENV ADDITIONAL_PACKAGES=${ADDITIONAL_PACKAGES}
+
+RUN \
+  touch /etc/s6-overlay/s6-rc.d/user/contents.d/init-custom-users \
+  && apt-get update \
+  && apt-get install --yes ${ADDITIONAL_PACKAGES}