Alpine Linuxにおけるapk updateの役割と必要性について知りたいです

前提

alpine Linuxベースのdockerfileを書いている際
npm installがうまくいかなかった。
これを受けてapk updateコマンドの詳細情報を検索してみたが見つけられなかった。

環境

docker-machine上にて環境を構築しています。

起こった問題のソース

RUN apk add --update nodejs npm; \
    npm install -g browserify; \
    npm install -g babel-cli; \
    apk add git;

↑こちらのソースではDokcerコンテナが立ち上がらなかった。
以下のように修正したらコンテナが立ち上がってきた。

RUN apk add --update nodejs npm; \
    apk update; \
    npm install -g browserify; \
    apk update; \
    npm install -g babel-cli; \
    apk update; \
    apk add git;

疑問点

alpine linux における apk update コマンドがやっていることを詳しく知りたいです。
また今回のようにapk updateを書く必要があるのはDockerの仕様によるものなのか
AlpineLinuxの仕様によるものなのかもわかりましたらお願いいたします。

質問内容

もっとスマートに書きたいのですが
有効情報にたどり着けていない状況です。
ヒントだけでもいただけましたら嬉しく思います。
よろしくお願いいたします。

テスト２回目結果

FROM nginx:mainline-alpine
RUN apk update; \
    apk add nodejs npm; \
    npm install -g browserify; \
    npm install -g babel-cli; \
    apk add git;
WORKDIR /app
EXPOSE 8080
CMD nginx -g "daemon off;"

version: '3.7'
services:
  webserver:
    build: ./
    container_name: nginx
    tty: true
    volumes:
      - ./app:/app
    ports:
      - "8080:80"

$ docker-compose build --no-cache
Building webserver
Step 1/5 : FROM nginx:mainline-alpine
mainline-alpine: Pulling from library/nginx
9d48c3bd43c5: Pull complete
b6dac14ba0a9: Pull complete
Status: Downloaded newer image for nginx:mainline-alpine
Step 2/5 : RUN apk update;     apk add nodejs npm;     npm install -g browserify;     npm install -g babel-cli;     apk add git;
fetch http://dl-cdn.alpinelinux.org/alpine/v3.10/main/x86_64/APKINDEX.tar.gz
fetch http://dl-cdn.alpinelinux.org/alpine/v3.10/community/x86_64/APKINDEX.tar.gz
v3.10.2-19 [http://dl-cdn.alpinelinux.org/alpine/v3.10/main]
v3.10.2-2 [http://dl-cdn.alpinelinux.org/alpine/v3.10/community]
OK: 10339 distinct packages available
(1/8) Installing ca-certificates (20190108-r0)
(2/8) Installing c-ares (1.15.0-r0)
(3/8) Installing libgcc (8.3.0-r0)
(4/8) Installing http-parser (2.9.2-r0)
(5/8) Installing libstdc++ (8.3.0-r0)
(6/8) Installing libuv (1.29.1-r0)
(7/8) Installing nodejs (10.16.3-r0)
(8/8) Installing npm (10.16.3-r0)
Executing busybox-1.30.1-r2.trigger
Executing ca-certificates-20190108-r0.trigger
OK: 82 MiB in 45 packages
npm ERR! code ENOTFOUND
npm ERR! errno ENOTFOUND
npm ERR! network request to https://registry.npmjs.org/cached-path-relative failed, reason: getaddrinfo ENOTFOUND registry.npmjs.org registry.npmjs.org:443
npm ERR! network This is a problem related to network connectivity.
npm ERR! network In most cases you are behind a proxy or have bad network settings.
npm ERR! network
npm ERR! network If you are behind a proxy, please make sure that the
npm ERR! network 'proxy' config is set properly.  See: 'npm help config'

npm ERR! A complete log of this run can be found in:
npm ERR!     /root/.npm/_logs/2019-08-28T12_45_46_775Z-debug.log
npm ERR! code ENOTFOUND
npm ERR! errno ENOTFOUND
npm ERR! network request to https://registry.npmjs.org/fs-readdir-recursive failed, reason: getaddrinfo ENOTFOUND registry.npmjs.org registry.npmjs.org:443
npm ERR! network This is a problem related to network connectivity.
npm ERR! network In most cases you are behind a proxy or have bad network settings.
npm ERR! network
npm ERR! network If you are behind a proxy, please make sure that the
npm ERR! network 'proxy' config is set properly.  See: 'npm help config'

npm ERR! A complete log of this run can be found in:
npm ERR!     /root/.npm/_logs/2019-08-28debug.log
(1/5) Installing nghttp2-libs (1.39.2-r0)
(2/5) Installing libcurl (7.65.1-r0)
(3/5) Installing expat (2.2.7-r0)
(4/5) Installing pcre2 (10.33-r0)
(5/5) Installing git (2.22.0-r0)
Executing busybox-1.30.1-r2.trigger
OK: 96 MiB in 50 packages
Removing intermediate container 
 ---> 
Step 3/5 : WORKDIR /app/reactbeginnersguide
 ---> Running in 
Removing intermediate container 0
 ---> 
Step 4/5 : EXPOSE 8080
 ---> Running in 
Removing intermediate container 
 ---> 
Step 5/5 : CMD nginx -g "daemon off;"
 ---> Running in 
Removing intermediate container 
 ---> 

Successfully built 
Successfully tagged react_webserver:latest

$docker-compose up -d --build
Creating network "react_default" with the default driver
Building webserver
Step 1/5 : FROM nginx:mainline-alpine
 ---> 
Step 2/5 : RUN apk update;     apk add nodejs npm;     npm install -g browserify;     npm install -g babel-cli;     apk add git;
 ---> Using cache
 ---> 
Step 3/5 : WORKDIR /app/react
 ---> Using cache
 ---> 
Step 4/5 : EXPOSE 8080
 ---> Using cache
 ---> 
Step 5/5 : CMD nginx -g "daemon off;"
 ---> Using cache
 ---> 

Successfully built 
Successfully tagged react_webserver:latest
Creating nginx ... done

$docker exec -it nginx /bin/sh
/app # browserify --version
/bin/sh: browserify: not found
/app # babel --version
/bin/sh: babel: not found