v0.18.21

What's Changed

• feat(train): support availability_model (spot/dedicated) on compute by @johnnt849 in #2520
• fix(ci): pin apt mirror for integration test image builds by @ndeepak-baseten in #2549
• feat(cli/train): split capacity view usage into On-Demand and Spot by @johnnt849 in #2547
• fix(image_builder): stop corrupting slim SERVER_START_CMD ENV values (RUN-564) by @ndeepak-baseten in #2552
• Release 0.18.21 by @basetenbot in #2555

Full Changelog: v0.18.20...v0.18.21