Add Amazon Linux 2 support, prepare for PG18

* Add Amazon Linux 2 as a supported OS.

* Dynamically select the package manager for Amazon and Oracle Linux.

* Prepare for supporting PostgreSQL 18.  Specifically, the Docker image
  seems to have moved the data directory, so get that dynamically.

docker/Dockerfile-amazonlinux

@@ -8,10 +8,11 @@ FROM ${DISTRO}:${RELEASE}
 ARG DISTRO
 ARG RELEASE
 
-RUN dnf install -y \
+RUN if [ ${RELEASE} -le 2 ] ; then MGR=yum ; else MGR=dnf ; fi && \
+    ${MGR} install -y \
     rpm-build \
     make \
-    && dnf clean all
+    && ${MGR} clean all
 
 RUN echo -e "#!/bin/bash\nmake actually-package-${DISTRO}-${RELEASE}" > /init.sh \
     && chmod 755 /init.sh

docker/Dockerfile-oraclelinux

@@ -8,11 +8,12 @@ FROM ${DISTRO}:${RELEASE}
 ARG DISTRO
 ARG RELEASE
 
-RUN yum install -y \
+RUN if [ ${RELEASE} -le 7 ] ; then MGR=yum ; else MGR=dnf ; fi && \
+    ${MGR} install -y \
     rpm-build \
     make \
-    oracle-epel-release-el7 \
-    && yum clean all
+    oracle-epel-release-el${RELEASE} \
+    && ${MGR} clean all
 
 RUN echo -e "#!/bin/bash\nmake actually-package-${DISTRO}-${RELEASE}" > /init.sh \
     && chmod 755 /init.sh