spack still concretizies spack.yaml with oneapi compiler instead
of gcc. This test builds with --no-cache option to rule out spack.yaml
caching.
dev.Dockerfile has a CACHE_BUST arg for this purpose but it seems to be
set after copying spack.yaml not before. So if this works, move
CACHE_BUST before COPY instruction copying spack.

Previous build fails because spack concretizes environment to use
oneapi-2022.0.0 compiler instead of gcc mentioned in spack.yaml
This test modifies spack.yaml to use intel compiler instead to check
if dependencies compile successfully.

python-is-python3 is unavaliable for Ubuntu 18.04 apparently.
apt-get update before apt-get install should take care of this, but
is removed for the time being.

Previous testing failed due to intel/oneapi-hpckit not being used
despite being the first command in the Dockerfile.
The most probable cause is the name of the Dockerfile being the same as
jug/dev.Dockerfile.

Another change is the addition of another Build stage to reduce
final image size.

Modified GitLab CI Config for a minimal test run