separate out gcc build stage for dev uses

Dockerfile

 # docker build --build-arg BORT=develop . -f shellb3-cspp-py3.Dockerfile -t shellb3-cspp-py3:latest
 # docker run -e SHELLB3_ARCHIVE_DIR=/dock -it --rm -v $HOME/dock:/dock -w /opt/shellb3-cspp-py3/ShellB3 shellb3-cspp-py3 /bin/bash -l -c "unset LANG; bash shallbethree.sh bin_dist"
-FROM centos:7
+FROM centos:7 as gcc
 RUN yum -y install gcc-gfortran gcc-c++ unzip perl patch byacc bison flex glibc-devel.i686 glibc-devel.x86_64 xz epel-release wget make bzip2 xz automake autoconf m4 which
 RUN yum -y install git git-lfs
 RUN test -d /opt || mkdir /opt
@@ -15,6 +15,8 @@ ARG TRIM=cspp
 ENV PORTABLESHELLB3=true
 ENV MAKEFLAGS=-j40
 RUN ./shallbethree.sh mk_gcc11
+
+FROM gcc as sb3
 ARG LINUXCPUFLAGS="-march=ivybridge -fPIC"
 ENV LINUXCPUFLAGS="${LINUXCPUFLAGS}"
 # numcodecs will break on avx2 build when ivybridge arch is forced

build-and-push.sh

 #!/bin/bash -e
+export REVNUM=GIT$(git rev-parse --verify HEAD --short)
+
 time docker build -t shellb3:latest \
     -t gitlab.ssec.wisc.edu:5555/cspp/buildbucket/shellb3/shellb3-centos7-gcc11-cspp-py3:r$REVNUM \
-    --build-arg REVNUM=GIT$(git rev-parse --verify HEAD --short) \
+    --build-arg REVNUM=$REVNUM \
     -f Dockerfile .
 
 docker push gitlab.ssec.wisc.edu:5555/cspp/buildbucket/shellb3/shellb3-centos7-gcc11-cspp-py3:r$REVNUM

build-gcc.sh

+#!/bin/bash -e
+export REVNUM=GIT$(git rev-parse --verify HEAD --short)
+
+time docker build -t shellb3-gcc:latest \
+    -t gitlab.ssec.wisc.edu:5555/cspp/buildbucket/shellb3/shellb3-gcc11:r$REVNUM \
+    --build-arg REVNUM=$REVNUM \
+    --target gcc \
+    -f Dockerfile .
+
+echo >&2 "==> gitlab.ssec.wisc.edu:5555/cspp/buildbucket/shellb3/shellb3-gcc11:r$REVNUM aka shellb3-gcc:latest"
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