How do I install bash shell in Alpine Linux?

It is easy to have bash installed but this does not mean the symlinks to busybox are gone. The syntax is as follows with apk command:
# apk update
# apk upgrade
# apk add bash
Sample outputs:

(1/4) Installing ncurses-terminfo-base (6.1_p20200118-r4)
(2/4) Installing ncurses-libs (6.1_p20200118-r4)
(3/4) Installing readline (8.0.1-r0)
(4/4) Installing bash (5.0.11-r1)
Executing bash-5.0.11-r1.post-install
Executing busybox-1.31.1-r9.trigger
OK: 11 MiB in 23 packages

Get a list of valid login shells on Alpine Linux using the cat command:
# cat /etc/shells
Outputs:

# valid login shells
/bin/sh
/bin/ash
/bin/bash

How to install bash man pages and other docs

To install bash documentation, enter:
# apk add bash-doc
To install bash automatic command line completion install, run:
# apk add bash-completion
Sample session:

How to set bash as login shell

To use bash as a shell just type bash:
$ bash
To login to alpine Linux LXD vm from host, enter:
$ lxc exec alpine-lxd-vm-name-here bash
One can change root shell to bash shell using the following method:
# vi /etc/passwd
Find user name and the default shell such as /bin/ash:

root:x:0:0:root:/root:/bin/ash 

Replace it with /bin/bash:

root:x:0:0:root:/root:/bin/bash

Customize bash shell

Here is a sample file displayed using the cat command
cat ~/.bashrc
Sample outputs:

alias update='apk update && apk upgrade'
export HISTTIMEFORMAT="%d/%m/%y %T "
export PS1='\u@\h:\W \$ '
alias l='ls -CF'
alias la='ls -A'
alias ll='ls -alF'
alias ls='ls --color=auto'
source /etc/profile.d/bash_completion.sh
export PS1="\[\e[31m\][\[\e[m\]\[\e[38;5;172m\]\u\[\e[m\]@\[\e[38;5;153m\]\h\[\e[m\] \[\e[38;5;214m\]\W\[\e[m\]\[\e[31m\]]\[\e[m\]\\$ "

See Customize the bash shell environments wiki page for more info.

How to find bash shell version

One can type the following command:
bash --version
Sample outputs:

GNU bash, version 5.0.11(1)-release (x86_64-alpine-linux-musl)
Copyright (C) 2019 Free Software Foundation, Inc.
License GPLv3+: GNU GPL version 3 or later 

This is free software; you are free to change and redistribute it.
There is NO WARRANTY, to the extent permitted by law.

Adding bash to Alpine Docker image

Add the following command to your Docker config file to install bash shell:
RUN apk add --no-cache bash

Conclusion

You learned how to install bash shell on Alpine Linux along with documents, man pages and configure the bash shell as per your needs. I suggest you read bash man pages/docs here online for more info.