Fix file ownership issues for self-hosted runners. (#141)

action/Dockerfile

@@ -1,6 +1,10 @@
 ARG IMAGE
 FROM $IMAGE
 
+ARG UNAME=runner
+ARG UID=1000
+ARG GID=1000
+
 LABEL "com.github.actions.name"="Unity - Builder"
 LABEL "com.github.actions.description"="Build Unity projects for different platforms."
 LABEL "com.github.actions.icon"="box"
@@ -10,9 +14,15 @@ LABEL "repository"="http://github.com/webbertakken/unity-actions"
 LABEL "homepage"="http://github.com/webbertakken/unity-actions"
 LABEL "maintainer"="Webber Takken <webber@takken.io>"
 
+RUN bash -c 'mkdir -p /github/{home,workflow,workspace}' && chown $UID:$GID -R /github/
+RUN getent group $GID || groupadd -g $GID $UNAME
+RUN id -u $UID &>/dev/null || useradd -m -u $UID -g $GID -s /bin/bash -d /github/home $UNAME
+
 ADD default-build-script /UnityBuilderAction
 ADD steps /steps
 RUN chmod -R +x /steps
 ADD entrypoint.sh /entrypoint.sh
 RUN chmod +x /entrypoint.sh
 ENTRYPOINT ["/entrypoint.sh"]
+
+USER $UID:$GID