build: better build tools-v2 and mount /etc/hosts

[zztaki]

What problem does this PR solve?

Issue Number: #2602 and #2978

Problem Summary:

1. tools-v2 is in the main process of compiling and cannot be skipped.
2. go1.12.8 dependency can't be downloaded due to resolving incorrect DNS address. Maybe you can see this.

What is changed and how it works?

What's Changed:

1. When only(akw. target) is specified when building, we will no longer compile tools-v2. (Warning: this will also happen if you manually specify all modules for only)
2. mount /etc/hosts to docker.

How it Works:

Side effects(Breaking backward compatibility? Performance regression?):

Check List

• [ ] Relevant documentation/comments is changed or added
• [x] I acknowledge that all my contributions will be made under the project's license

[zztaki]

cicheck