Merge ENV statements to reduce amount of layers

[felipecrs]

If we see for example the documentation about the Node script, it says:

ENV NVM_DIR="/usr/local/share/nvm"
ENV NVM_SYMLINK_CURRENT=true \
    PATH=${NVM_DIR}/current/bin:${PATH}

But it could be simplified to:

ENV NVM_DIR="/usr/local/share/nvm" \
    NVM_SYMLINK_CURRENT=true \
    PATH=${NVM_DIR}/current/bin:${PATH}

In order to reduce the amount of the layers in the Docker image.

This could possibly be applied to all the Dockerfiles and the scripts docs.

[Chuxel]

@felipecrs This doesn't work because NVM_DIR is used in the path, and therefore is not defined in the second example. Did this first and reverted because of it.

[felipecrs]

You're totally right, I tested before and I thought that it worked in the same ENV statement, but now I tested again and it does not work.

Sorry for the confusion.