Cannot install bash in nginx:alpine

Question

I have the following Dockerfile:

# => Build container
FROM node:14-alpine AS builder
WORKDIR /app

# split the dependecies from our source code so docker builds can cache this step
# (unless we actually change dependencies)
COPY package.json yarn.lock ./
RUN yarn install
COPY . ./
RUN yarn build

# => Run container
FROM nginx:alpine

# Nginx config
WORKDIR /usr/share/nginx/html
RUN rm -rf ./*
COPY ./nginx/nginx.conf /etc/nginx/conf.d/default.conf

# Default port exposure
EXPOSE 8080

# Copy .env file and shell script to container
COPY --from=builder /app/dist .
COPY ./env.sh .
COPY .env .

USER root

# Add bash
RUN apk update && apk add --no-cache bash

# Make our shell script executable
RUN chmod +x env.sh

# Start Nginx server
CMD ["/bin/bash", "-c", "/usr/share/nginx/html/env.sh && nginx -g \"daemon off;\""]

But when I try to run it I get the following errors when trying to add bash:

------
 > [stage-1 8/9] RUN apk update && apk add --no-cache bash:
#19 0.294 fetch https://dl-cdn.alpinelinux.org/alpine/v3.14/main/x86_64/APKINDEX.tar.gz
#19 0.358 140186175183688:error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1914:
#19 0.360 fetch https://dl-cdn.alpinelinux.org/alpine/v3.14/community/x86_64/APKINDEX.tar.gz
#19 0.360 ERROR: https://dl-cdn.alpinelinux.org/alpine/v3.14/main: Permission denied
#19 0.360 WARNING: Ignoring https://dl-cdn.alpinelinux.org/alpine/v3.14/main: No such file or directory
#19 0.405 140186175183688:error:1416F086:SSL routines:tls_process_server_certificate:certificate verify failed:ssl/statem/statem_clnt.c:1914:
#19 0.407 ERROR: https://dl-cdn.alpinelinux.org/alpine/v3.14/community: Permission denied
#19 0.407 WARNING: Ignoring https://dl-cdn.alpinelinux.org/alpine/v3.14/community: No such file or directory
#19 0.408 2 errors; 42 distinct packages available
------
executor failed running [/bin/sh -c apk update && apk add --no-cache bash]: exit code: 2

Which I need to add to execute this script:

#!/bin/bash

# Recreate config file
rm -rf ./env-config.js
touch ./env-config.js

# Add assignment 
echo "window._env_ = {" >> ./env-config.js

# Read each line in .env file
# Each line represents key=value pairs
while read -r line || [[ -n "$line" ]];
do
  # Split env variables by character `=`
  if printf '%s\n' "$line" | grep -q -e '='; then
    varname=$(printf '%s\n' "$line" | sed -e 's/=.*//')
    varvalue=$(printf '%s\n' "$line" | sed -e 's/^[^=]*=//')
  fi

  # Read value of current variable if exists as Environment variable
  value=$(printf '%s\n' "${!varname}")
  # Otherwise use value from .env file
  [[ -z $value ]] && value=${varvalue}
  
  # Append configuration property to JS file
  echo "  $varname: \"$value\"," >> ./env-config.js
done < .env

echo "}" >> ./env-config.js

From what I have read adding USER root should fix this but it does not in this case.

Any ideas on how to fix?

Answer 1

You should be able to use /bin/sh as a standard Bourne shell; also, you should be able to avoid the sh -c wrapper in the CMD line.

First, rewrite the script using POSIX shell syntax. Scanning over the script, it seems like it is almost okay as-is; change the first line to #!/bin/sh, and correct the non-standard ${!varname} expansion (also see Dynamic variable names in Bash)

# Read value of current variable if exists as Environment variable
value=$(sh -c "echo \$$varname")
# Otherwise use value from .env file
[[ -z $value ]] && value=${varvalue}

You can try testing it using an alpine or busybox image with a more restricted shell, or setting the POSIXLY_CORRECT environment variable with bash.

Secondly, there's a reasonably standard pattern of using ENTRYPOINT and CMD together. The CMD gets passed as arguments to the ENTRYPOINT, so if the ENTRYPOINT ends with exec "$@", it will replace itself with that command.

#!/bin/sh
#      ^^ not bash

# Recreate config file
...
echo "window._env_ = {" >> ./env-config.js
...
echo "}" >> ./env-config.js

# Run the main container CMD
exec "$@"

Now, in your Dockerfile, you don't need to install GNU bash, because you're not using it, but you do need to correctly split out the ENTRYPOINT and CMD.

ENTRYPOINT ["/usr/share/nginx/html/env.sh"] # must be JSON-array syntax
CMD ["nginx", "-g", "daemon off;"]          # can be either syntax

As an aside, the cmd1 && cmd2 syntax is basic shell syntax and not a bash extension, so you could write CMD ["/bin/sh", "-c", "cmd1 && cmd2"]; but, if you write a command without the JSON array syntax, Docker will insert the sh -c wrapper for you. You should almost never need to write out sh -c in a Dockerfile.