skeema / skeema / 5649721901

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docker: use Docker CLI instead of API

This commit replaces the implementation of Skeema's Docker interactions.
Previously, Skeema used Docker's API via a Golang Docker client library. This
commit switches to shelling out to the `docker` CLI, instead of bundling a
Docker client package and its dependencies.

For background, Skeema interacts with Docker in two places:

1. The workspace=docker option, which uses ephemeral Docker containers for
   all temporary scratch-space database operations

2. Integration test suites, which extensively test Skeema end-to-end against
   containerized databases of different flavors/versions

This commit affects both of these touch-points. Item 1 affects end users,
while item 2 only affects Skeema developers.

Motivations for this change include:

* 40% reduction in Skeema Community Edition binary size, due to massive
  reduction in Skeema's third-party package dependency graph

* Prevention of spurious security warnings: the previous Docker client's own
  dependencies often have CVEs, despite the relevant code paths not being used
  in that Docker client, let alone in Skeema

* Potential compatibility with alternative container engines which provide
  daemonless rootless drop-in replacements for the `docker` binary

However, there is one user-facing downside to this change: users who run
`skeema` inside a container environment, while also using workspace=docker,
may have to adjust their containers to ensure the `docker` CLI binary is now
present. This is in addition to the previous complexities discussed in #89:
either bind-mounting the Docker socket, or using a privileged Docker-in-Docker
image. The `docker` CLI binary can easily be obtained from the docker:cli
image or from a package manager.

RFC / tracking issue #186 has been open for a year, and has not yielded
significant community concerns, so this downside is deemed acceptable.

In the Premium Edition, the temp-schema-environment option also presents an
altern... (continued)

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