Update all of the Dockerfiles to not use yum update

Author: shahmishal

5.2/amazonlinux/2/slim/Dockerfile

@@ -29,7 +29,7 @@ RUN set -e; \
     && gpg --batch --quiet --keyserver ha.pool.sks-keyservers.net --recv-keys "$SWIFT_SIGNING_KEY" \
     && gpg --batch --verify swift.tar.gz.sig swift.tar.gz \
     # - Unpack the toolchain, set libs permissions, and clean up.
-    && yum -y update && yum -y install tar gzip \
+    && yum -y install tar gzip \
     && tar -xzf swift.tar.gz --directory / --strip-components=1 $SWIFT_VERSION-$SWIFT_PLATFORM/usr/lib/swift/linux \
     && chmod -R o+r /usr/lib/swift \
     && rm -rf "$GNUPGHOME" swift.tar.gz.sig swift.tar.gz \

nightly-master/amazonlinux/2/slim/Dockerfile

@@ -34,7 +34,7 @@ RUN set -e; \
     && curl -fSsL https://swift.org/keys/all-keys.asc | gpg --import -  \
     && gpg --batch --verify latest_toolchain.tar.gz.sig latest_toolchain.tar.gz \
     # - Unpack the toolchain, set libs permissions, and clean up.
-    && yum -y update && yum -y install tar gzip \
+    && yum -y install tar gzip \
     && tar -xzf latest_toolchain.tar.gz --directory / --strip-components=1 ${DOWNLOAD_DIR}-${OS_VER}/usr/lib/swift/linux \
     && chmod -R o+r /usr/lib/swift \
     && rm -rf "$GNUPGHOME" latest_toolchain.tar.gz.sig latest_toolchain.tar.gz \

swift-ci/master/amazon-linux/2/Dockerfile

@@ -1,6 +1,6 @@
 FROM amazonlinux:2
 
-RUN yum -y update && yum install shadow-utils.x86_64 -y
+RUN yum install shadow-utils.x86_64 -y
 
 RUN groupadd -g 998 build-user && \
     useradd -m -r -u 42 -g build-user build-user
@@ -10,7 +10,6 @@ RUN groupadd -g 998 build-user && \
 RUN yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-7.noarch.rpm
 
 # Update and install needed build packages
-RUN yum -y update
 RUN yum -y group install "development tools"
 RUN yum -y install \
   clang            \

swift-ci/master/centos/7/Dockerfile

@@ -1,6 +1,6 @@
 FROM centos:7
 
-RUN yum -y update && yum install shadow-utils.x86_64 -y
+RUN yum install shadow-utils.x86_64 -y
 
 RUN groupadd -g 998 build-user && \
     useradd -m -r -u 42 -g build-user build-user
@@ -48,4 +48,4 @@ WORKDIR /home/build-user
 
 ENV LC_ALL en_US.UTF-8
 ENV LANG en_US.UTF-8
-ENV LANGUAGE en_US.UTF-8
+ENV LANGUAGE en_US.UTF-8

swift-ci/master/centos/8/Dockerfile

@@ -5,8 +5,6 @@ RUN groupadd -g 42 build-user && \
 
 RUN yum install -y https://dl.fedoraproject.org/pub/epel/epel-release-latest-8.noarch.rpm
 
-RUN yum -y update
-
 RUN yum install --enablerepo=PowerTools -y \
     autoconf              \
     clang                 \