ci: Rename openj9 Dockerfiles to hotspot

We used to pull AdoptOpenJDK's OpenJ9 builds but switched to
Eclipse Temurin, which uses the HotSpot VM instead of OpenJ9.
Rename the corresponding Dockerfiles to hotspot.

Signed-off-by: Younes Manton <ymanton@ca.ibm.com>

scripts/build/Dockerfile.hotspot-alpine

@@ -1,6 +1,4 @@
-# FIXME: Replace with eclipse-temurin once Alpine support has been added.
-# https://github.com/adoptium/containers/pull/60
-FROM adoptopenjdk/openjdk8-openj9:alpine
+FROM docker.io/library/eclipse-temurin:8-alpine
 ARG CC=gcc
 
 RUN apk update && apk add \
@@ -29,4 +27,3 @@ WORKDIR /criu
 RUN make mrproper && make -j $(nproc) CC="$CC"
 
 ENTRYPOINT mvn -q -f test/javaTests/pom.xml test
-