issues found by shellcheck were:
```
$ shellcheck docker-entrypoint.sh
In docker-entrypoint.sh line 10:
if [[ ${USERMAP_UID} != ${USERMAP_ORIG_UID} || ${USERMAP_GID} != ${USERMAP_ORIG_GID} ]]; then
^-- SC2053: Quote the rhs of != in [[ ]] to prevent glob matching.
^-- SC2053: Quote the rhs of != in [[ ]] to prevent glob matching.
In docker-entrypoint.sh line 12:
groupmod -g ${USERMAP_GID} paperless
^-- SC2086: Double quote to prevent globbing and word splitting.
In docker-entrypoint.sh line 65:
if dpkg -s "$pkg" 2>&1 > /dev/null; then
^-- SC2069: The order of the 2>&1 and the redirect matters. The 2>&1 has to be last.
In docker-entrypoint.sh line 69:
if ! apt-cache show "$pkg" 2>&1 > /dev/null; then
^-- SC2069: The order of the 2>&1 and the redirect matters. The 2>&1 has to be last.
```
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