Fix tilt file cache_from, optimise dockerfile (#3264)

# What this PR does

## Which issue(s) this PR fixes

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [ ] Documentation added (or `pr:no public docs` PR label added if not
required)
- [ ] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

Tiltfile

@@ -35,8 +35,7 @@ allow_k8s_contexts(["kind-kind"])
 docker_build_sub(
     "localhost:63628/oncall/engine:dev",
     context="./engine",
-    cache_from="grafana/oncall:latest",
-    # only=["./engine", "./grafana-plugin"],
+    cache_from=["grafana/oncall:latest"],
     ignore=["./grafana-plugin/test-results/", "./grafana-plugin/dist/", "./grafana-plugin/e2e-tests/"],
     child_context=".",
     target="dev",

engine/Dockerfile

@@ -17,7 +17,6 @@ RUN apk add bash \
 
 WORKDIR /etc/app
 COPY ./requirements.txt ./
-COPY ./pip/cache /root/.cache/pip/wheels/
 RUN pip install -r requirements.txt
 
 # we intentionally have two COPY commands, this is to have the requirements.txt in a separate build step