Run linter against shell scripts

[xlgmokha]

Why is this needed?

To ensure that we follow the best practices for shell scripting described in
the Shellcheck wiki. e.g. SC2006.

What does this do?

It adds a ./bin/lint script that can be used to run the shellcheck linter
on your machine. It also adds a job to the CI workflow to ensure that the
linter checks are passing.

xref: #3917 (comment)

Screenshots

Before:

モ ./bin/lint

In ./bin/docker-dev-shell line 14:
OPTS=`getopt -o hr --long help,rebuild -n 'parse-options' -- "$@"`
^--^ SC2034: OPTS appears unused. Verify use (or export if used externally).
     ^-- SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean:
OPTS=$(getopt -o hr --long help,rebuild -n 'parse-options' -- "$@")


In ./bin/docker-dev-shell line 15:
if [ $? != 0 ]; then
     ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.


In ./bin/docker-dev-shell line 61:
echo $RUNNING
     ^------^ SC2086: Double quote to prevent globbing and word splitting.

Did you mean:
echo "$RUNNING"


In ./hex/helpers/build line 18:
case `uname` in
     ^-----^ SC2006: Use $(...) notation instead of legacy backticked `...`.

Did you mean:
case $(uname) in

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- OPTS appears unused. Verify use (...
  https://www.shellcheck.net/wiki/SC2086 -- Double quote to prevent globbing ...
  https://www.shellcheck.net/wiki/SC2006 -- Use $(...) notation instead of le...

After:

モ ./bin/lint
~/src/github.com/xlgmokha/dependabot-core [turtles-in-a-half-shellcheck]
モ echo $?
0

Type of change

• Bug fix (non-breaking change which fixes an issue)

[xlgmokha]

I added the ignores above to skip the following:

モ ./bin/lint

In ./bin/docker-dev-shell line 14:
OPTS=$(getopt -o hr --long help,rebuild -n 'parse-options' -- "$@")
^--^ SC2034: OPTS appears unused. Verify use (or export if used externally).


In ./bin/docker-dev-shell line 15:
if [ $? != 0 ]; then
     ^-- SC2181: Check exit code directly with e.g. 'if mycmd;', not indirectly with $?.

For more information:
  https://www.shellcheck.net/wiki/SC2034 -- OPTS appears unused. Verify use (...
  https://www.shellcheck.net/wiki/SC2181 -- Check exit code directly with e.g...

[jurre]

I had one question about installing the new dependency in the dev container

[jurre]

Ah, ok automerge was enabled. I suppose we can add shellcheck to the dev container in a separate PR