fix: increase HTTP 500 retry count to 5 with longer backoff and visible logging

The Gitea registry's offset mismatch race condition needs more than
~15s to recover. Increase retries from 3 to 5 and backoff from 5-20s
to 10-60s exponential, giving up to ~2 minutes of recovery time.
Add before_sleep logging so retries are visible in CI output.

# Implements: REQ-1, REQ-2, REQ-3

docs/specs/DEVX-164.md

@@ -0,0 +1,34 @@
+# DEVX-164: Increase HTTP 500 retry count and backoff for docker push
+
+## Problem
+The HTTP 500 retry logic (DEVX-162, DEVX-163) works correctly — 3 retry
+attempts are made. But all 3 attempts fail because the Gitea registry's
+"offset mismatch" race condition needs more than ~15s to recover. The
+current backoff is 5s-20s with 3 attempts (total ~15s of waiting).
+
+## Approach
+Increase retry count from 3 to 5 and backoff from 5-20s to 10-60s,
+giving the registry up to ~2 minutes to recover. Add visible logging
+between retry attempts so the CI logs show the retry happening.
+
+REQ-1: Increase retry count from 3 to 5
+REQ-2: Increase backoff from 5-20s to 10-60s exponential
+REQ-3: Add visible logging between retry attempts (click.echo)
+REQ-4: All tests pass with 100% coverage
+
+## Test Plan
+- Unit tests verify retry count and backoff parameters
+- Unit tests verify logging output on retry
+- Manual: trigger build-images workflow and verify retries visible in logs
+
+## Deploy Plan
+- Merge to master
+
+## Rollback Plan
+- Revert the merge commit
+
+## Acceptance Criteria
+- [x] REQ-1: Increase retry count from 3 to 5
+- [x] REQ-2: Increase backoff from 5-20s to 10-60s exponential
+- [x] REQ-3: Add visible logging between retry attempts (click.echo)
+- [x] REQ-4: All tests pass with 100% coverage