Fix Dockerfile editor settings writing (#158)

The script relies on Bash-specific features, so Bash must
be used instead of POSIX sh or dash.

mono.Dockerfile

@@ -2,6 +2,7 @@ FROM mono:latest
 LABEL author="https://github.com/aBARICHELLO/godot-ci/graphs/contributors"
 
 USER root
+SHELL ["/bin/bash", "-c"]
 ENV DEBIAN_FRONTEND=noninteractive
 RUN apt-get update && apt-get install -y --no-install-recommends \
     ca-certificates \