contrib: Dockerfile: add editorconfig-checker

Co-authored-by: Jan-Niklas Burfeind <git@aiyionpri.me>
version bump to 2.6.0
aligned indentation

contrib/docker/Dockerfile

@@ -24,6 +24,12 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     && apt-get clean \
     && rm -rf /var/lib/apt/lists/*
 
+RUN mkdir /tmp/ec &&\
+    wget -O /tmp/ec/ec-linux-amd64.tar.gz https://github.com/editorconfig-checker/editorconfig-checker/releases/download/2.6.0/ec-linux-amd64.tar.gz &&\
+    tar -xvzf /tmp/ec/ec-linux-amd64.tar.gz &&\
+    mv bin/ec-linux-amd64 /usr/local/bin/editorconfig-checker &&\
+    rm -rf /tmp/ec
+
 RUN useradd -d /gluon gluon
 USER gluon