fix: inherit LABELS, only set `title`
This PR combines and inherits LABEL definitions and helps avoid large blocks of LABELS in the docker history output, i.e.:
<missing> About an hour ago ARG TARGETPLATFORM=linux/amd64 0B buildkit.dockerfile.v0
<missing> About an hour ago COPY /opt/spack-environment/test/spack.* /op… 43.4kB buildkit.dockerfile.v0
<missing> About an hour ago RUN |2 TARGETPLATFORM=linux/amd64 ENV=test d… 126B buildkit.dockerfile.v0
<missing> About an hour ago ENV SPACK_ENV=/opt/spack-environment/test 0B buildkit.dockerfile.v0
<missing> About an hour ago ARG ENV=test 0B buildkit.dockerfile.v0
<missing> About an hour ago COPY . /opt/spack-environment/ # buildkit 16kB buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.license=GPL-3… 0B buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.documentation… 0B buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.source=https:… 0B buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.url=https://g… 0B buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.authors=Elect… 0B buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.vendor=Electr… 0B buildkit.dockerfile.v0
<missing> About an hour ago LABEL org.opencontainers.image.title=Electro… 0B buildkit.dockerfile.v0
<missing> About an hour ago ARG TARGETPLATFORM=linux/amd64 0B buildkit.dockerfile.v0
<missing> 15 hours ago RUN |10 TARGETPLATFORM=linux/amd64 SPACK_ORG… 89B buildkit.dockerfile.v0
<missing> 15 hours ago ADD https://github.com/eic/eic-spack.git#8b6… 226kB buildkit.dockerfile.v0
<missing> 15 hours ago ARG EICSPACK_VERSION=8b6346d8880318084dc6335… 0B buildkit.dockerfile.v0