Check the CONTAINER_TOOL value and use it in the e2e test script

[hdefazio]

Description

• Add a more robust check for the check-container-tool make target to catch empty CONTAINER_TOOL values
• Change the run_e2e.sh script to use the CONTAINER_TOOL value, defaulting to docker if it is unset

How Has This Been Tested?

$ make check-container-tool 
✅ Container tool 'podman' found.

(this is correct for my system)

$ make test-e2e
✅ Container tool 'podman' found.
==== Building Docker image ghcr.io/llm-d/llm-d-inference-scheduler:dev ====
podman build \

Other cases:
Manually set CONTAINER_TOOL := ""

$ make check-container-tool 
❌ Error: No container tool detected. Please install docker or podman.
make: *** [Makefile:297: check-container-tool] Error 1

Manually set CONTAINER_TOOL := "docker"

$ make check-container-tool 
❌ Error: 'docker' is not installed or not in your PATH.
🔧 Try: sudo apt install docker OR brew install docker
make: *** [Makefile:297: check-container-tool] Error 1

(this is correct for my system)

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

Summary by CodeRabbit

• New Features

  • Allow selection of container tool via CONTAINER_TOOL, propagated to sub-commands.
  • Added check-builder target to validate and report the selected builder.

• Improvements

  • Enhanced container tool checks with clearer validation and guidance when the tool is missing or not in PATH.

• Tests

  • E2E script now honors CONTAINER_TOOL (defaulting to docker), prints the chosen tool, and uses it for image operations.

[pierDipi]

/lgtm

/cc @nirrozenbaum

[shmuelk]

Please rebase this PR