Rewrote license activation to use a lock to prevent race-conditions.

unity-command/Dockerfile

@@ -1,11 +1,19 @@
 #Arg is replaced with the desired Unity container.
-ARG IMAGE=unityci/editor:2023.1.16f1-windows-mono-3
+ARG IMAGE=ubuntu:latest
 FROM ${IMAGE}
 
+ARG DOCKER_VERSION=20.10.23
+
 RUN apt-get update
 RUN apt-get install -y software-properties-common apt-transport-https wget chromium-browser
 RUN add-apt-repository -y ppa:savoury1/chromium
-RUN apt-get update && apt-get install -y chromium-browser
+RUN apt-get update && \
+    apt-get install -y chromium-browser && \
+    apt-get install -y curl
+
+# Install Docker CLI (static binary - much cleaner)
+RUN apt-get update && apt-get install -y curl
+RUN curl -fsSL https://download.docker.com/linux/static/stable/x86_64/docker-${DOCKER_VERSION}.tgz | tar -xzf - --strip=1 -C /usr/local/bin docker/docker
 
 #ADD https://minio.studiowhy.net/hackmd/UnityBuilder /usr/local/bin/
 COPY UnityBuilder /usr/local/bin/
@@ -15,10 +23,5 @@ COPY entrypoint.sh /
 RUN chmod +x /entrypoint.sh
 
 COPY scripts/. /scripts
-# Commented out until better image selection is enabled.
-# RUN --mount=type=secret,id=SERIAL \
-#     --mount=type=secret,id=USERNAME \
-#     --mount=type=secret,id=PASSWORD \
-#     bash /scripts/activate_license.sh
 
 ENTRYPOINT ["/entrypoint.sh"]