Unable to locate package language-pack-en

Question

On my NodeJS server (running as docker image)...

Dockerfile

FROM node:4.8-slim
RUN apt-get update -y && \
    apt-get install -yqq locales git

...there is no english locale installed:

RUN locale -a

gives me

C
C.UTF-8
POSIX

In my dockerfile I try to add the missing language

RUN apt-get install -y language-pack-en

But this gives me

Reading package lists...
Building dependency tree...
Reading state information...
E: Unable to locate package language-pack-en

So how can I add the missing language pack?

---

Update

Using

sudo locale-gen en_US
sudo locale-gen en_US.UTF-8
sudo update-locale 

gives me this error:

perl: warning: Setting locale failed.
perl: warning: Please check that your locale settings:
    LANGUAGE = "en_US:en",
    LC_ALL = "en_US.UTF-8",
    LANG = "en_US.UTF-8"
    are supported and installed on your system.
perl: warning: Falling back to the standard locale ("C").
locale: Cannot set LC_CTYPE to default locale: No such file or directory
locale: Cannot set LC_MESSAGES to default locale: No such file or directory
locale: Cannot set LC_COLLATE to 

default locale: No such file or directory

Answer 1

If it's a debian image, you can add the following in your Dockerfile

RUN apt-get install -y locales locales-all
ENV LC_ALL en_US.UTF-8
ENV LANG en_US.UTF-8
ENV LANGUAGE en_US.UTF-8