Fix - install ps, which is necessary for starting Bloop

[lwronski]

The latest docker image version, debian:stable-slim, doesn't include ps. I added the installation of ps to ensure bloop starts correctly; without it, the following error was thrown during Bloop's initialization:

 Exception in thread "main" java.io.IOException: Cannot run program "ps": error=2, No such file or directory
 	at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1143)
 	at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1073)
 	at scala.sys.process.ProcessBuilderImpl$Simple.run(ProcessBuilderImpl.scala:75)
 	at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang(ProcessBuilderImpl.scala:119)
 	at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.slurp(ProcessBuilderImpl.scala:135)
 	at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang$bang(ProcessBuilderImpl.scala:108)
 	at libdaemonjvm.internal.Processes$.isRunning(Processes.scala:20)
 	at libdaemonjvm.internal.LockProcess$Default.$anonfun$isRunning$2(LockProcess.scala:16)
 	at scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.java:23)
 	at scala.Option.getOrElse(Option.scala:189)
 	at libdaemonjvm.internal.LockProcess$Default.isRunning(LockProcess.scala:16)
 	at libdaemonjvm.server.Lock$.$anonfun$tryAcquire$6(Lock.scala:76)
 	at libdaemonjvm.server.Lock$.$anonfun$tryAcquire$6$adapted(Lock.scala:75)
 	at scala.Option.flatMap(Option.scala:271)
 	at libdaemonjvm.server.Lock$.ifFiles$1(Lock.scala:75)
 	at libdaemonjvm.server.Lock$.tryAcquire(Lock.scala:94)
 	at bloop.Bloop$.loop$1(Bloop.scala:95)
 	at bloop.Bloop$.main(Bloop.scala:113)
 	at bloop.Bloop.main(Bloop.scala)
 Caused by: java.io.IOException: error=2, No such file or directory
 	at java.base/java.lang.ProcessImpl.forkAndExec(Native Method)
 	at java.base/java.lang.ProcessImpl.<init>(ProcessImpl.java:314)
 	at java.base/java.lang.ProcessImpl.start(ProcessImpl.java:244)
 	at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1143)
 	at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1073)
 	at scala.sys.process.ProcessBuilderImpl$Simple.run(ProcessBuilderImpl.scala:75)
 	at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang(ProcessBuilderImpl.scala:119)
 	at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.slurp(ProcessBuilderImpl.scala:135)
 	at scala.sys.process.ProcessBuilderImpl$AbstractBuilder.$bang$bang(ProcessBuilderImpl.scala:108)
 	at libdaemonjvm.internal.Processes$.isRunning(Processes.scala:20)
 	at libdaemonjvm.internal.LockProcess$Default.$anonfun$isRunning$2(LockProcess.scala:16)
 	at scala.runtime.java8.JFunction0$mcZ$sp.apply(JFunction0$mcZ$sp.java:23)
 	at scala.Option.getOrElse(Option.scala:189)
 	at libdaemonjvm.internal.LockProcess$Default.isRunning(LockProcess.scala:16)
 	at libdaemonjvm.server.Lock$.$anonfun$tryAcquire$6(Lock.scala:76)
 	at libdaemonjvm.server.Lock$.$anonfun$tryAcquire$6$adapted(Lock.scala:75)
 	at scala.Option.flatMap(Option.scala:271)
 	at libdaemonjvm.server.Lock$.ifFiles$1(Lock.scala:75)
 	at libdaemonjvm.server.Lock$.tryAcquire(Lock.scala:94)
 	at bloop.Bloop$.loop$1(Bloop.scala:95)
 	at bloop.Bloop$.main(Bloop.scala:113)
 	at bloop.Bloop.main(Bloop.scala)
 Caused by: java.io.IOException: error=2, No such file or directory
 	at java.base/java.lang.ProcessImpl.forkAndExec(Native Method)
 	at java.base/java.lang.ProcessImpl.<init>(ProcessImpl.java:314)
 	at java.base/java.lang.ProcessImpl.start(ProcessImpl.java:244)
 	at java.base/java.lang.ProcessBuilder.start(ProcessBuilder.java:1110)

[tgodzik]

LGTM, though would be good not to require ps to exist at all