I noticed arm-image-installer's shebang was to /bin/sh and ran shellcheck on the script and it found a bunch of bashisms.
Here is a list of them on the current master branch (669bbf7bc) in the format line_number: problem:

• 129: The surrounding quotes actually unquote this. Remove or escape them.
• 175: In POSIX sh, read -p is undefined.
• 176: Quote parameters to tr to prevent glob expansion.
• 176: Quote parameters to tr to prevent glob expansion.
• 188: Prefer [ p ] && [ q ] as [ p -a q ] is not well defined.
• 270: In POSIX sh, read -p is undefined.
• 271: Quote parameters to tr to prevent glob expansion.
• 271: Quote parameters to tr to prevent glob expansion.
• 277: In POSIX sh, &> is undefined.
• 279: Quote this to prevent word splitting.
• 279: In POSIX sh, &> is undefined.
• 350: In POSIX sh, standalone ((..)) is undefined.
• 350: In POSIX sh, ++ is undefined.
• 362: In POSIX sh, standalone ((..)) is undefined.
• 362: In POSIX sh, ++ is undefined.
• 381: In POSIX sh, &> is undefined.
• 387: In POSIX sh, standalone ((..)) is undefined.
• 387: In POSIX sh, ++ is undefined.
• 396: In POSIX sh, &> is undefined.
• 405: In POSIX sh, &> is undefined.
• 406: In POSIX sh, &> is undefined.
• 409: In POSIX sh, &> is undefined.
• 410: In POSIX sh, &> is undefined.
• 422: In POSIX sh, brace expansion is undefined.
• 422: In POSIX sh, &> is undefined.
• 423: In POSIX sh, &> is undefined.
• 436: In POSIX sh, &> is undefined.
• 441: In POSIX sh, &> is undefined.
• 470: In POSIX sh, [[ ]] is undefined.
• 484: Quote parameters to tr to prevent glob expansion.
• 484: Quote parameters to tr to prevent glob expansion.
• 488: Quote parameters to tr to prevent glob expansion.
• 488: Quote parameters to tr to prevent glob expansion.
• 511: In POSIX sh, &> is undefined.
• 515: In POSIX sh, &> is undefined.
• 535: In POSIX sh, [[ ]] is undefined.
• 571: In POSIX sh, &> is undefined.
• 573: In POSIX sh, &> is undefined.
• 575: In POSIX sh, &> is undefined.
• 586: The surrounding quotes actually unquote this. Remove or escape them.