fixtures: fix read-only krb5kdc composeBuild failure (#8799)

Running composeBuild task for krb5kdc-fixture throws read-only file
system error while writing hostname to local hosts file. Since,
hostname resolution is already present and pointing to local node,
let's remove writing it again from the Dockerfile.

See: #8761

Signed-off-by: Rohit Ashiwal <rashiwal@amazon.com>

test/fixtures/krb5kdc-fixture/Dockerfile

@@ -2,7 +2,7 @@ FROM ubuntu:14.04
 RUN apt update -y
 RUN apt upgrade -y
 ADD . /fixture
-RUN echo kerberos.build.opensearch.org > /etc/hostname && echo "127.0.0.1 kerberos.build.opensearch.org" >> /etc/hosts
+RUN echo kerberos.build.opensearch.org > /etc/hostname
 RUN bash /fixture/src/main/resources/provision/installkdc.sh
 
 EXPOSE 88