- Both FetchContainer and DockerBuild read the image metadata and
convert to LLB (e.g. `ENV foo bar` in Dockerfile shows up in
`op.#Exec`)
- Image metadata is "sticky" between Pipelines (e.g. `op.#Load` will
re-use the same metadata)
- Image metadata is injected back to #PushContainer, so that
DockerBuild+PushContainer and FetchContainer+PushContainer do not lose
any metadata.
- Tests for all the above
Fixes#142
Signed-off-by: Andrea Luzzardi <aluzzardi@gmail.com>