Incorrect command for building ARM64 plugins for on-premise

[SafinWasi]

The command for building custom plugins for on-premise ARM64 installations is incorrect. In addition, the code header is also incorrect, as it says "Build your plugin for Alpine ARM64" for both Alpine and non-Alpine.

https://www.krakend.io/docs/extending/writing-plugins/#plugin-builder

Incorrect command:

docker run -it -v "$PWD:/app" -w /app \
-e "CGO_ENABLED=1" \
-e "CC=aarch64-linux-musl-gcc" \
-e "GOARCH=arm64" \
-e "GOHOSTARCH=amd64" \
krakend/builder:2.9.1-linux-generic \
go build -ldflags='-extldflags=-fuse-ld=bfd -extld=aarch64-linux-musl-gcc' \
-buildmode=plugin -o yourplugin.so .

Correct command:

docker run -it -v "$PWD:/app" -w /app \
-e "CGO_ENABLED=1" \
-e "CC=aarch64-linux-gnu-gcc" \
-e "GOARCH=arm64" \
-e "GOHOSTARCH=amd64" \
krakend/builder:2.9.1-linux-generic \
go build -ldflags='-extldflags=-fuse-ld=bfd -extld=aarch64-linux-gnu-gcc' \
-buildmode=plugin -o yourplugin.so .

aarch64-linux-musl-gcc only exists on alpine images. For the generic image aarch64-linux-gnu-gcc must be used.

[alombarte]

@SafinWasi thanks for reporting this inconsistency!