Previously, images produced by 'guix pack -f docker' would always show
up as "profile" in the output of 'docker images'. With this change,
'docker images' shows a name constructed from the packages found in the
image--e.g., "bash-coreutils-grep-sed".
* guix/docker.scm (canonicalize-repository-name): New procedure.
(generate-tag): Remove.
(manifest): Add optional 'tag' parameter and honor it.
(repositories): Likewise.
(build-docker-image): Add #:repository parameter and pass it to
'manifest' and 'repositories'.
* guix/scripts/pack.scm (docker-image)[build]: Compute 'tag' and pass it
as #:repository to 'build-docker-image'.
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