fix: skip Docker Hub network calls when Dolt image not cached

[PabloLION]

Summary

• Check for locally cached Docker image before calling dolt.Run(), which contacts Docker Hub even when the image is already present
• Tests now skip with a helpful docker pull command when the exact image:tag is missing
• Gates all 4 entry points: StartTestDoltServer, StartIsolatedDoltContainer, EnsureDoltContainerForTestMain, RequireDoltContainer

Fixes #2277

Behavior matrix

Docker	Image cached	Cache state	Result
Stopped	—	—	Skip: "Docker not available"
Running	No	—	Skip: "run docker pull dolthub/dolt-sql-server:1.43.0"
Running	Yes	Version matches	Starts container (no network)
Running	Yes	Wrong version	Skip: "run docker pull dolthub/dolt-sql-server:1.43.0"

Rows 2 and 4 produce the same outcome — docker image inspect checks the exact tag, so a stale cached version won't match.

Test plan

• Docker stopped → tests skip with "Docker not available" (unchanged)
• Docker running + exact image cached → container starts normally (no network)
• Docker running + image not cached → tests skip with pull instruction
• Docker running + wrong version cached → tests skip with correct version in message
• go vet ./internal/testutil/ passes
• go test ./cmd/bd/... -count=1 passes (pre-existing TestCheckTestPollution_NoTestIssues failure unrelated)

🤖 Generated with Claude Code

[PabloLION]

Closing in favor of c2888b1 which takes a cleaner approach — merging the image cache check into isDockerAvailable() instead of gating 4 call sites separately. Single point of change, cached result, and the BEADS_SKIP_DOCKER=1 escape hatch is a nice bonus.