update the dockerfile gen command

[mengqiy]

• Split go mod download as a separate step to avoid download the go modules every single time when the users make non-dependency changes to the source code.
• Using ENTRYPOINT instead of CMD, so the users doesn't need to know what command they need to run (e.g. If the command is not called function, the user can still run docker run my-image-name --help)
• Dropped -tags netgo, since it's not necessary when we already set CGO_ENABLED=0.
• Added -s in -ldflags to disable symbol table and it further reduces the image size.

[mengqiy]

/cc @KnVerey

[KnVerey]

Makes sense, thanks!
/approve
/lgtm

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