Less verbosity in build logs (netty#15049)

Motivation:
We don't particularly need thousands of lines of output from commands
that are just setting up the build environment.

Modification:
- Remove verbose flags from tar commands; we don't need to know every
single file extracted from these archives.
- Tell SDKMAN to use a default config suitable for CI; this will make it
less verbose when installing java versions.

Result:
Less build output in our logs that isn't relevant to our actual maven
build.

Author: chrisvest

docker/Dockerfile.centos6

@@ -22,7 +22,7 @@ RUN yum install -y \
 
 
 # Downloading and installing SDKMAN!
-RUN curl -s "https://get.sdkman.io" | bash
+RUN curl -s "https://get.sdkman.io?ci=true" | bash
 
 ARG java_version="8.0.302-zulu"
 ENV JAVA_VERSION $java_version

docker/Dockerfile.cross_compile_aarch64

@@ -19,7 +19,7 @@ WORKDIR $SOURCE_DIR
 
 # Install aarch64 gcc 10.2 toolchain
 RUN wget https://developer.arm.com/-/media/Files/downloads/gnu-a/$GCC_VERSION/binrel/gcc-arm-$GCC_VERSION-x86_64-aarch64-none-linux-gnu.tar.xz && \
-   tar xvf gcc-arm-$GCC_VERSION-x86_64-aarch64-none-linux-gnu.tar.xz && mv gcc-arm-$GCC_VERSION-x86_64-aarch64-none-linux-gnu /opt/
+   tar xf gcc-arm-$GCC_VERSION-x86_64-aarch64-none-linux-gnu.tar.xz && mv gcc-arm-$GCC_VERSION-x86_64-aarch64-none-linux-gnu /opt/
 ENV PATH="/opt/gcc-arm-$GCC_VERSION-x86_64-aarch64-none-linux-gnu/bin:${PATH}"
 
 ENV JAVA_HOME="/usr/lib/jvm/java-1.8.0-openjdk/"