Add support for passing secure env vars and runtime BuildGraph args under Linux

This is primarily useful for configuring UBA when building the engine.

This initial support can be used when generating Dockerfiles to be built extenerally. Plumbing for using UBA when running `ue4-docker build` will be added later.

Author: adamrehn

src/ue4docker/dockerfiles/ue4-minimal/linux/Dockerfile

@@ -23,16 +23,18 @@ RUN if [ -f ./Engine/Build/BatchFiles/BuildUBT.sh ]; then \
 	fi
 
 # Create an Installed Build of the Engine
+ARG BUILDGRAPH_ARGS=""
 WORKDIR /home/ue4/UnrealEngine
-RUN {% if automatic_build_id %}export BUILD_ID_OVERRIDE=UE_`cat ./Engine/Build/Build.version | jq --raw-output '"\(.MajorVersion).\(.MinorVersion)"'` && {% endif %}\
+RUN {% if build_secret_vars %}{% for var in build_secret_vars %}--mount=type=secret,id={{ var }},env={{ var }} {% endfor %}{% endif %}\
+	{% if automatic_build_id %}export BUILD_ID_OVERRIDE=UE_`cat ./Engine/Build/Build.version | jq --raw-output '"\(.MajorVersion).\(.MinorVersion)"'` && {% endif %}\
 	./Engine/Build/BatchFiles/RunUAT.sh BuildGraph \
 	-target="Make Installed Build Linux" \
 	-script=Engine/Build/InstalledEngineBuild.xml \
 	-set:HostPlatformOnly=true \
 	-set:WithClient=true \
 	-set:WithDDC={% if excluded_components.ddc == true %}false{% else %}true{% endif %} \
 	-set:WithServer=true \
-	{%+ if automatic_build_id %}-set:BuildIdOverride="$BUILD_ID_OVERRIDE" {% endif %}{{ buildgraph_args }} && \
+	{%+ if automatic_build_id %}-set:BuildIdOverride="$BUILD_ID_OVERRIDE" {% endif %}{{ buildgraph_args }} ${BUILDGRAPH_ARGS} && \
 	rm -R -f /home/ue4/UnrealEngine/LocalBuilds/InstalledDDC
 
 # Ensure UnrealVersionSelector is built, since the prebuilt binaries may not be up-to-date