refactor(docker): remove obsolete Dockerfile for Ubuntu 18.04

* Deleted the `Dockerfile.ubuntu-1804` as it is no longer needed.
* This change helps to streamline the Docker setup and reduce clutter.

Dockerfile.ubuntu-1804

@@ -1,37 +0,0 @@
-FROM ubuntu:18.04
-
-LABEL maintainer="LinuxGSM <me@danielgibbs.co.uk>"
-ARG DEBIAN_FRONTEND=noninteractive
-
-SHELL ["/bin/bash", "-o", "pipefail", "-c"]
-
-# Install SteamCMD
-RUN echo "**** Install SteamCMD ****" \
-  && echo steam steam/question select "I AGREE" | debconf-set-selections \
-  && echo steam steam/license note '' | debconf-set-selections \
-  && dpkg --add-architecture i386 \
-  && apt-get update \
-  && apt-get install -y --no-install-recommends ca-certificates locales lib32gcc1 libsdl2-2.0-0:i386 tzdata steamcmd \
-  && ln -s /usr/games/steamcmd /usr/bin/steamcmd \
-  && apt-get -y autoremove \
-  && apt-get -y clean \
-  && rm -rf /usr/share/man /usr/share/doc /usr/share/info /usr/share/lintian /usr/share/locale/* \
-  && rm -rf /var/lib/apt/lists/* /tmp/* /var/tmp/*
-
-# Add unicode support
-RUN locale-gen en_US.UTF-8
-ENV LANG=en_US.UTF-8
-ENV LANGUAGE=en_US:en
-
-# Create non-root user
-RUN useradd -m -d /home/steam -s /bin/bash steam \
-  && mkdir -p /home/steam/Steam \
-  && chown -R steam:steam /home/steam
-USER steam
-WORKDIR /home/steam
-
-ENTRYPOINT ["steamcmd"]
-CMD ["+help", "+quit"]
-
-# Basic health check: run a no-op steamcmd help then quit
-HEALTHCHECK --interval=30m --timeout=30s --start-period=30s --retries=3 CMD steamcmd +login anonymous +app_info_update 1 +quit > /dev/null 2>&1 || exit 1